Gelatin is a gelling agent, of animal origin, which is used in cooking to thicken or set preparations. It dissolves when heated (at about 60°C), and then acts as a gelling agent when the temperature drops again. If you intend to buy: Gelatin can be found in supermarkets in 2 gram sheets or as a powder.
Be aware that sometimes you can find it in smaller sheet of 1 gram instead of 2, so you should note that recipes here use full size sheets of 2 grams. If you want to use it: See this page.