The greenhouse effect is the natural heating of earth caused by the atmosphere trapping heat.
How does this effect work?
There are 5 greenhouse gases: Carbon Dioxide, Methane, Water Vapor, Nitrous Oxide, and Flourinated Gases!
Now the greenhouse effect is how the earth's atmosphere is trapping heat using the gases, so the earth doesn't freeze. Later on the earth lets out a little bit of heat, so the earth doesn't burn or become the sun's twin!
So it's like an actual greenhouse just for the planet.