The Earth is warmed by the Sun. The Sun gives out mostly short wavelength radiation, which passes through the atmosphere and is absorbed by the Earth's surface. The warm surface then gives out radiation of its own. Because the surface is much cooler than the Sun, this radiation has a long wavelength and is infrared.
Some gases in the atmosphere absorb this long wavelength radiation instead of letting it escape to space. They then re-radiate it in all directions, including back towards the Earth, which keeps the surface warmer. This is called the greenhouse effect. The gases that cause it are the greenhouse gases: water vapour, carbon dioxide and methane.
Without greenhouse gases, the Earth would be much colder and far too cold for most life. They keep temperatures high enough to support life.
