Photosynthesis is what makes our lives possible. If you have studied entropy already, you know that you have to constantly put energy into a system if you want to maintain or increase order.

The source of almost all that energy is the sun. Puny animals like us can't use the sun's energy directly, we need plants to turn that energy into food. Photo (light) synthesis (making) basically means making food from the energy in light.

Plants take CO₂ (carbon dioxide) and water (H₂O) and make sugar (C₆H₁₂O₆) and O₂. (To have a balanced equation you'd have to say that 6CO₂+6H₂O yield C₆H₁₂O₆+6O₂.) Note that plants are taking small molecules and building a bigger one. That requires energy. The energy comes from photons (units of light) that are captured by pigments like chlorophyll; the stuff that makes plants green.

The actual steps are rather complicated, but the idea is to slowly release the energy in one photon, use that energy to make ATP (the cell's "universal power source," then use ATP to make the sugar. The oxygen is just a byproduct, but it sure comes in handy.

When we break down food to release the energy we need, we are doing the opposite of the equation above. That's why we require oxygen. Plants do this too. They make sugars when they can, then burn them when it's dark.