A bellows is a bag-like device with handles that's used to blow air onto a fire to keep the flame burning. If you have a fireplace in your house, you might have a bellows too.
The kind of bellows that provides extra oxygen to a dwindling fire is similar to other types of bellows: they all involve a bag of air. Various instruments use a bellows, to provide a constant flow of air. Accordions, some organs, and bagpipes all have this kind of bellows. In the 13th century, the word was belwes, literally "bags," from the Old English belg, "bag or purse."