The airlock sits on top of any fermenter vessel and allow carbon dioxide produced by the active yeast to escape without allowing potentially infectious bacteria and wild yeast into the fermenter. Airlocks usually sell for about $1 a piece. In addition to the airlock, you'll need a rubber stopper, shown on the bottom of each airlock, to fit the size of the hole cut from the top of your fermenter. Rubber stoppers sell for under $1 each.If you don't use an airlock, the carbon dioxide (CO2) made by the yeast will build up in pressure and blow the lid off.
The airlock is a "S" shaped bit of tubing that holds some water in it - outgoing gases can bubble out through the liquid, but nothing tends to find its way in. (sometimes a very curious fly will have a fatal go at it...)
Where to get it