How does the greenhouse effect work?

The greenhouse effect is when radiation from a planet's atmosphere warms the planet's surface to a temperature higher than what it would be without its atmosphere. If a planet's atmosphere has radiatively active gases the atmosphere will radiate energy everywhere

Gases in the atmosphere such as carbon dioxide do what the roof of a green house does. During the day the sunshines through the atmosphere. Earths surface warms up in the sunlight. At night, Earths surface cools, releasing the heat back into the air.