Plants produce their own food (sugars) using carbon dioxide, water, and sunlight. Oxygen is a by-product of this reaction.
Then, we (or herbivores or other omnivores) eat the plants to get their sugar and breathe in oxygen which creates energy (for living), as well as by-products of carbon dioxide and water.
The chemistry gets far more complex than this! :}"I may not have gone where I intended to go, but I think I have ended up where I needed to be." ~ Douglas Adams
