What does each item used in the experiment represent in the actual greenhouse effect on earth? for instance, the light used in the experiment represents the sun. what does the bottom of the box, air in the box, and plastic wrap each represent in the greenhouse effect?

The light represents the sunlight.

The bottom of the box represents the surface of the Earth.

The air in the box represents the atmosphere.

The plastic wrap represents the part of the atmosphere where the Greenhouse gases are and influence the temperature.

So we have a box, air in it, plastic wrap, and directed light on it. The bottom of the box represents the surface of the Earth. The light that is directed towards it gets partially through the plastic wrap, reaching the bottom of the box and warming it up. As it warms up, the bottom of the box starts to radiate heat and returns it upwards through the air that represents the atmosphere. Then that heat reaches the plastic wrap, but it is not able to move through it, thus the air starts to warm up more and more, giving us a nice simplified example of how this process works.