It's the plastic thing on this mason jar...keeps the air out so you don't end up with vinegar. The one I have is in a rubber stopper that fits in the top of the jug.
Or you can make your own by punching a hole in the bottle lid and putting in a piece of tubing so that one end is in the bottle headspace and the other end is sumerged in water. (seal around the lid with wax)
Make that “submerged”