Popular sovereignty is a basic idea of democracy. Popular sovereignty means that the people are the ultimate source of the authority of their government. Popular sovereignty means that democratic government is BY THE PEOPLE AND FOR THE PEOPLE—for the benefit of the people, not for the benefit of those who govern in their name.
Popular sovereignty is the belief that the legitimacy of the government is created by the will or consent of its People, who's natural rights are the source of all political power. The idea was first expressed by John Locke, developed and written by George Mason in the Virginia Declaration of Rights in June of 1776, used by Thomas Jefferson in writing the Declaration of Independence, and used by Benjamin Franklin and others in writing Constitutions for many of the newly declared States.
Virginia Declaration of Rights
A Declaration of Rights, made by the representatives of the good people of Virginia, assembled in full Convention; and recommended to Posterity as the Basis and Foundation of Government.
That all Men  are born equally free and independent, and have certain inherent natural Rights, of which they can not by any Compact, deprive or divest their Posterity; among which are the Enjoyment of Life and Liberty, with the means of acquiring and possessing Property, and pursuing and obtaining Happiness and Safety. That Power is, by God and Nature, vested in, and consequently derived from the People; that Magistrates are their Trustees and Servants, and at all times amenable to them.
That Government is, or ought to be, instituted for the common Benefit and Security of the People, Nation, or Community. Of all the various Modes and Forms of Government, that is best, which is capable of producing the greatest Degree of Happiness and Safety, and is most effectually secured against the Danger of mal-administration. And that whenever any Government shall be found inadequate, or contrary to these Purposes, a Majority of the Community had an indubitable, inalienable and indefeasible right to reform, alter or abolish it, in such Manner as shall be judged most conducive to the Public Weal.
That no man or set of men are entitled to exclusive or separate emoluments or privileges from the community, but in consideration of public services; which not being descendible, or hereditary, the Idea of a man born a magistrate, a legislator, or a judge is unnatural and absurd.
Declaration of Independence
"We hold these truths to be self-evident, that all men are created equal, that they are endowed by their Creator with certain unalienable Rights, that among these are Life, Liberty and the pursuit of Happiness.
That to secure these rights, Governments are instituted among Men, deriving their just powers from the consent of the governed, That whenever any Form of Government becomes destructive of these ends, it is the Right of the People to alter or to abolish it, and to institute new Government, laying its foundation on such principles and organizing its powers in such form, as to them shall seem most likely to effect their Safety and Happiness."- Declaration of Independence
The idea was first expressed by philosopher John Locke, who wrote much of the material that the Jefferson based the document upon. It is the foundation of the act of separation, as the British American colonies had been induced to taxes and acts by Parliament, and King George III that began to undermine their pursuit of Happiness and Liberty, and life as well. They, therefore, rose up and abolished the British rule and "instituted new government".
You will probably notice that the Declaration of Independence and the Virginia Declaration bear remarkable similarity. The Virginia Declaration draft states that "all men are born equally free and independent" as opposed to "all men are created equal" in the Declaration of Independence, and the amended Virginia Declaration was expressly adopted as the basis and foundation of Government, whereas the Declaration of Independence does not possess that power. Before the end of the Revolutionary War, most US States had adopted declarations similar to Virginia's. While many of the delegates to the Constitutional Convention were from states that objected to the original "born free and independent" language of Mason's draft (they were from slave states) nearly all believed that their State Documents would ensure that the new Federal Government would have to preserve promote and protect individual rights as a part of the common good.
Mr. Mason anticipated that there would come a time when the New Federal Government would attempt to subvert those rights unless they were expressly protected. He attempted to get the Convention to adopt language acknowledging universal individual rights as the basis for the US Constitution and guaranteeing the sanctity of those rights. He wasn't able to get all he wanted but was able to prompt the adoption of the US Bill of Rights as amendments to the US Constitution. Still, some of the clarity of liberal intent of his original draft was lost, and considerable confusion has become a part of our heritage. We Liberals may desire that the clear declaration of rights in Mr. Mason's original document were part of our Constitution, but we understand that even Mr. Mason's adversaries accepted those rights (at least for white male property owners) as a given premise to Popular Government.
Difference with Nationalism, Patriotism
Patriotism can be taken to the level that one is so blind as to believe in one's country no matter what it is doing. With it, the bloc of people will forget the purpose of government, namely, to protect its people's Life, Liberty, and Happiness. For example, the USA PATRIOT Act undermines the American people's liberty, and some their happiness. Therefore, according to popular sovereignty, when after a long train of abuses and usurpations does the government keeps undermining its cause and purpose, the people should alter it, regardless of bureaucracy. At the very least the people should vote for rulers who won't abuse power.
In a democracy the People OWN THE GOVERNMENT. It is theirs and they have the right to change it by lawful, constitutional methods if it does not perform to their satisfaction. 
Liberals tend to advocate this method of governing for society, as many of the American Revolutionaries were liberal. Conservatism, after all, is about conserving the status quo and current systems, which leads many conservatives to be against popular sovereignty. Liberals attempt to change if the state is repeatedly undermining its purpose, whereas conservatives would proclaim liberals traitors, in a manner of speaking, and against their country. This is due to another one of the ideas expressed in the Declaration of Independence:
"all experience hath shewn, that mankind are more disposed to suffer, while evils are sufferable, than to right themselves by abolishing the forms to which they are accustomed"- Declaration of Independence
Not everyone can be as open-minded as a liberal. It takes a true visionary to see through a cloud of usurpation. Conservatives have trouble interpreting and understanding facts, even when such facts would benefit them. Perhaps this is why they supported President George W. Bush even when he was undermining Americans' civil liberties. Sadly Obama's record over civil liberties isn't faultless either.
- European Convention on Human Rights
- Universal Declaration of Human Rights