Photosynthesis happens in green plants. It is the way they use energy from sunlight to turn carbon dioxide and water into food. 

 Photosynthesis is very important. Most forms of life on Earth depend on it  for energy. Plants obviously do - they photosyntheise food for their own  immediate needs. But, animals which eat plants and the animals who eat animals who eat plants also depend on photosynthesis. If this process did not fix carbon dioxide into sugar, whole food chains would collapse and life 
 would cease to exist.