Carbon dioxide is a gas which exists in large quantities in the air. It is released by humanoid life forms and most animals as a byproduct of respiration, the process used to create energy for the cells in the body to function. The Oxygen used to create the Carbon Dioxide comes from the air, and so if there is nothing to turn the Carbon Dioxide back into Oxygen for re-use, then the Oxygen supply will run out.
Most plants and some micro-organisms convert Carbon Dioxide back into oxygen, creating a fairly constant balance of Carbon Dioxide and Oxygen on most planets. Without them, any airtight space (such as a starship) will require an artificial air regeneration system to keep atmosphere breathable, and this usually forms part of the Life support system. All known starships have a Life Support system, even small ships such as Puddle Jumpers.