If the rotation axis of a planet is tilted, then the northern hemisphere of the planet will sometimes be tilted towards the sun, and at other times the southern hemisphere will be tilted towards the sun. The hemisphere that is tilted towards the Sun receives more light because more of that hemisphere is facing the Sun. So that half of the planet is a little bit warmer than the other half. This effect leads to seasons. The Greenhouse Effect Radiative equilibrium is a simple model that takes into account light reflected from the surface of a planet and the light that is absorbed by that surface.
Atmospheres can complicate the model.
Certain gases, like H 2 O and CO 2 are transparent to visible light and opaque to infrared light. The visible light coming from the sun goes through the atmosphere and hits the surface. The part that is absorbed is emitted as infrared light. Instead of simply being radiated into space, this infrared light cannot travel through the H 2 O and CO 2 in the atmosphere. It is absorbed by these gases and emitted again as heat.