Here's a trick question: Name the single most important thing needed for life on earth. (And no, the answer is not "Wii.") Give up? It's sunlight. The sun provides every living thing with the three basic necessities of life on earth: food, water, and oxygen. Obviously, we can't eat or drink sunlight itself. Nor can we breathe it in. So how do we get access to the life-sustaining nourishment from the sun? That's where photosynthesis comes in.
Photosynthesis is a chemical process whereby plants and algae that contain chlorophyll capture radiant energy from the sun, and use carbon dioxide and water from the environment to then convert the sunlight to food (glucose), while at the same time creating oxygen and water as byproducts.
Of course, we humans get the benefit of all that food, water, and yummy oxygen that photosynthesis creates. At the same time, we contribute to the process in a continuous cycle of dependencies. We expel lots of carbon dioxide into the environment in the process of breathing, which provides some of the necessary components for photosynthesis to create more food, oxygen, and water. Everything's connected in the process: The sun shines, we breathe, plants suck up sunlight and carbon dioxide, which in turn is used to create food and water for us to eat and drink, and oxygen for us to breathe... And the cycle continues for as long as the sun shines.