The green house affect is much like a car on a hot day. If you park you car in an open area on a hot day, the car will get hot inside. This happens because the heat comes in through the windows and windshield, bounces off of a surface, an cannot get back out. This happens with the Earth's atmosphere as well. The energy from the sun comes in through Earth's outer layer of atmosphere. It then cannot eacape back out, thus creating global warming.
Hope this helped!
*I did a project on this in 6th grade