Men, woman and children breath air as a nature function which enables them to live. If we just seal ourselves in a room we will be using up this oxygen with our normal breathing.
Ventilation is the free entry and circulation of air in a space such as a room.
A vent is an opening allowing air to pass through. Ventilation is required if the air in a room is being used up by flames burning, people breathing and so on. If ventilation is not provided then the existing oxygen in the air will be used up, the flames will start to smoke and eventually go out and the people will lapse into unconsciousness and, eventually suffocate. It is not enough to assume that a flue, if fitted, will take out the stale air. A separate high level vent is essential - particularly if people are likely to be in the room.