What is Greenhouse effect?

Greenhouse gasses absorb thermal radiation from Earth atmosphere and re-radiate them in every possible directions. These thermal radiation comes back to Earth surface and there by increases the average atmospheric temperature of Earth surface. In simple word we can think Greenhouse gasses act as a cover on top of Earth atmosphere, allows heat to comes down to Earth surface and block the heat going back to outside of Earth's atmosphere and therefore increases the atmospheric temperature of Earth surface. This whole process is known as Greenhouse effect.