John Maynard Keynes was a 20th Century British Liberal economist who wrote much of the Liberal Party manifesto for David Lloyd George's 1929 election campaign. Unfortunately the Liberals lost the election, but the ideas were taken up by FDR in his New Deal a few year later; and became economic orthodoxy for the United States of America and several other European countries a generation after World War II.
The essence of Keynesian economics was that Governments could mitigate the boom bust cycle by borrowing and spending more in the Economic downturn and repaying those debts in the upturn.  This reduces the pain of recessions and takes some of the craziness out of economic bubbles.
Note repaying debts during good times is an important part of Keynsianism. When Greece and a few other European Nations borrowed and spent too much without repaying later this caused major economic crises.
Conservative Arguments against Keynes and Keynesianism
Most Liberals recognize Keynes as the most important economist of the 20th Century. Most conservatives find it difficult to accept Keynes as both a leading intellectual and bisexual.