Photosynthesis is the process by which green plants, algae, and some bacteria convert light energy, usually from the sun, into chemical energy stored in glucose. During this process, plants take in carbon dioxide (CO2) from the air and water (H2O) from the soil. Using sunlight as energy, these raw materials are transformed into glucose (C6H12O6) and oxygen (O2).
The glucose produced is used by the plant as a source of energy and for growth. The oxygen, however, is released into the atmosphere as a byproduct. This oxygen is essential for the survival of most living organisms, including humans, as it is used in cellular respiration to produce energy.
Therefore, the waste product of photosynthesis is oxygen. While it is a byproduct for the plant, it is a vital component for other life forms on Earth.