I believe that what it means is that for each molecule of CO2 that the plant takes in, it breaks the C and O2 bond to use the carbon in its food-production process. The leftover O2 molecule is “waste” for the photosynthesis process, but that doesn’t mean that all of the O2 that the plant doesn’t utilize immediately is “released” as most animals release most CO2 when they breathe. (In fact, I don’t think you can exhale “all” of the waste CO2 when you breathe. Some is always left in the lungs. I expect that something similar occurs with plants, except for the “lungs” thing, obviously.)