A demijohn is a large glass flagon that is stoppered with a rubber or plastic bung and an airlock. They are used in the alcoholic fermentation of fruit and the like. They are also called carboys and come in a range of sizes.
My one has an old school s-type air bubble lock that after inserting in the bung I simply fill with boiling water (you only have to do this once) to keep every hygienic and the bugs out 🙂 . As the fermentation process slows, the pressure on the airlock reduces and a nice layer of carbon dioxide forms, in the demijohn, to preserve the contents.
Tip: Never fill a cold glass vessel with boiling water without warming it in a bath of hot water from the tap. It will crack otherwise.