Greenhouse Effect
Greenhouse Effect is how trapped air in a closed space warms up faster than open air around it.
Think of it this way
A closed glass jar left in the sun traps heat inside its walls. Light passes through the glass, but the warmth cannot escape back out. The air inside the jar gets much hotter than the air just outside it. This is how Earth's air traps the sun's heat close to the ground.
Explaining greenhouse effect by grade level
A greenhouse is a small house made of clear walls. The sun heats the air inside. That warm air cannot leave, so it stays hot. A plant in a greenhouse grows more than one left outside.