Used for sealing your fermenting vessel and allowing CO2 to vent.
Please note that grommets are sold separately.
Here is a video with different types.
This S-bubble airlock is the preferred choice for most fermenters, as it allows you to monitor the pressure in the fermenter. Fill to line with boiled (and cooled) water or sanitiser to create a closed system, which allows CO2 to escape, but doesn’t allow air back in to contaminate your wine or beer.