In the process of photosynthesis, plants (containing chlorophyll) use carbon dioxide, water and light energy and produce chemical energy (in the form of sugars) and oxygen. The balanced chemical reaction is given as:
`6CO_2 + 6H_2O + (light) -> C_6H_(12)O_6 + 6O_2`
This reaction is significant for a number of reasons. Photosynthesis provides us food (in the form of sugars and plat parts). It maintains the oxygen levels in our atmosphere (lack of which will kill aerobic life forms life us) and get rid of excess carbon dioxide (excess of which may lead to global warming and climate change).
Interestingly, we (human beings) and other higher life forms (including animals) use an opposite chemical reaction for our survival: we consume food and oxygen, and produce carbon dioxide and water (no light energy).
Hope this helps.