Imagine breathing through a 50-foot garden hose.

You breathe in, you might take in all the air in the hose.

You breathe out, you put that same air back into the hose.

Breathe in again, and you get the same air you breathed out but now it doesn't have much oxygen left.

By the third breath, your lungs have removed all the oxygen out of the air in the hose. The volume of air in the hose is considered "dead air" (tidal volume).