Photosynthesis is how plants harness the radiant energy from sunlight. This process is important since it's originally how all of the energy in fossil fuels (except for the abiogenic fossil fuels) was captured. This process is also indirectly how animals and humans get energy from the sun in the form of food.

The levels of atmospheric oxygen also come from photosynthesis, since that's how oxygen is chemically liberated from carbon dioxide to allow the atmosphere of Earth to support life.

Photosynthesis is incredibly important to life, and worth learning more about; two good places to start are hyperphysics (particularly nice is the discussion of how photosynthesis allows plants to appear to overcome the second law of thermodynamics by converting disorder into order) and UC Davis's chem wiki, which has an extensive overview of photosynthesis.