Some gases in the atmosphere keep the Earth warm enough for life. These are greenhouse gases. Three of them are water vapour, carbon dioxide and methane. Without them the average surface temperature would be far lower and life as we know it could not survive.
The greenhouse effect works through radiation. The Sun gives out short wavelength radiation, which passes through the atmosphere and is absorbed by the Earth's surface. The warm surface then emits radiation back out, but this is long wavelength radiation.
Greenhouse gases absorb some of this long wavelength radiation instead of letting it all escape to space. They then re-radiate it in all directions, and some of it goes back towards the Earth's surface. This keeps the surface and the lower atmosphere warmer than they would otherwise be.
