This 3 piece airlock or fermentation lock is filled partially with liquid (water/sanitizer) and it sticks in the top of a drilled stopper and onto the top of your fermentor. It allows the fermentation gasses to escape without allowing anything to contaminate your fermentor. The benefit of the 3 piece design is that it is much easier to clean than traditional S or Bubble type air locks. The grommet allows for easy installation into a bucket lid and provides a good seal. You should always have plenty of these on hand. This listing is for 1 complete airlock and grommet.