Plain English Meaning
A small bag on the oxygen mask that catches the air you breathe out and lets you breathe some of it back in, so the oxygen that wasn't used the first time isn't wasted.
Full Definition
A flexible reservoir bag attached to a continuous-flow oxygen mask that captures the user's exhaled breath from the upper airway and combines it with incoming oxygen, allowing the unused oxygen from the previous breath to be inhaled again on the next breath.
Context Anchor
Seen on continuous-flow oxygen masks used in unpressurized aircraft at higher altitudes.
Derivation
From 're-' (again) and 'breathe' -- literally a bag for breathing again. The name describes the function: exhaled air is held in the bag and re-inhaled rather than being lost.
Why Pilots Care
It conserves the aircraft’s limited , extending the time the can be used before the is depleted.