Solar Energy (sunlight) is short-wavelength radiation which easily penetrates the Earth's atmosphere and warms the Earth; only about one quarter of incoming sunlight is reflected by the atmosphere. The warmed Earth emits long-wavelength radiation (infrared waves or heat energy) back into space; these longer waves are mostly reflected back to Earth by the atmosphere. 
Greenhouse gasses are transparent to sunlight but are relatively opaque to the infrared radiation that the Earth radiates out into space. Therefore sunlight gets through the Earth’s atmosphere and warms the Earth. When the Earth radiates the heat outwards as infrared radiation greenhouse gasses reflect part of the radiation and send it back to Earth. The process is complicated and readers who are keen to learn it can look at our links below.
Basically the most important fact that you need to know is that the Earth is unnaturally warm because of greenhouse gasses that humans put into our atmosphere.