What do plants breath out?

Plants have a nearly opposite respiration with that of humans. They harness the energetic photons of sunlight along with the ability to fix carbon dioxide (CO2) in separate reactions to create carbohydrates (R-6-P) as the product and, to answer your question, Oxygen as the respiratory "waste" product. This fact is why you often hear people say that we need trees and plants to breathe and sustain oxygen levels at their optimal P02 levels of around 25-28%