They burn energy from ATP molecules and produce carbon dioxide. Then they wait for diffusion of oxygen to enter and 'recharge the ADP molecules back into ATP molecules so they can burn them again. As a simpler way to say it: In tissue respiration carbon dioxide is released and oxygen is re-absorbed.
This the word equation for celluar respiration:
C6H12O6 + 6O2 -------------------> 6CO2 + 6H2O + ~38 ATP