They are organic self-replicating collectors of solar energy. Now, if making all the food on the planet was all that trees and plants did that would be quite a lot. However there is more. We burn the sugar we get from plants by combining it with oxygen in our bodies to release the energy we need to live. You've heard about burning calories.
Calories are a measure of heat.
When we say there are 16 calories in a teaspoon of sugar we are saying a teaspoon of sugar will give off 16 calories of heat when burned. Most of us know that without oxygen, nothing will burn. We also know that without oxygen, we will die. We now have it down to a few basics. Trees and plants collect the energy from the sun and convert it into sugar and other tasty foods.