When it comes to fermentation, it's important to keep outside environment outside without forcing carbon dioxide to build up in the container. An airlock accomplishes just that. Using liquid as an interface, all of undesirables from the room are kept out (oxygen, insects, pet hair), while still allowing CO2 to escape.
The holes in the drilled stoppers are sized for the airlock that we sell. Each stopper's product description lists its dimensions to help you pick the right one.