The airlock (or fermentation lock) is a fairly vital piece of equipment for home winemaking. Pour some potassium metabisulphite solution, water or vodka in it, insert it in the top of your fermenter and it allows the carbon dioxide to escape whilst your wine is fermenting, whilst maintaining a barrier to keep contaminants out.
Product may differ slightly from picture depending on supplier.
Choose from the s-bend or bubbler airlock or the cylindrical style (aka senior airlock). The senior airlock style may suit you if you have limited height to store your wine as it protrudes less far than the s-bend style. (about 10cm above the bung compared to about 16cm for the s-bend).