Here's a breakdown:

* Photosynthesis: This is the process where plants use sunlight, water, and carbon dioxide to create their own food (glucose) and oxygen.

* Oxygen as a byproduct: The oxygen is a byproduct of this process, meaning it's a substance produced as a result of photosynthesis, but not the main purpose of the process.

So, plants are essentially "breathing out" oxygen, which is essential for the survival of many organisms, including humans.