Bubbler Airlock with Cap
Bubbler Airlock. Also has a plastic cap to keep dust and insects out. Fitted to the lid of the fermenter using a rubber bung, the airlock holds a small amount of water (see imager in a U bend that will allow carbon dioxide to escape from the fermenter but will not allow air back in. Air can oxidize your beer or wine so this is important especially if the fermentation takes more than a week or two. Airlocks also come with a small "cap" that keeps fruit flies out.
Do not overfill the airlock; otherwise stale water can get sucked into your fermenter.