Pectin is a natural occurring substance found in some fruits and vegetables. This substance is used to thicken jams, jellies and preserves. Certain fruits like quince have so much natural pectin that it does not require anything to thicken it. But other fruits like apples, pears do need something to thicken the mixture. Pectin does require sugar and acid in order to thicken. Make sure to follow your recipe and be cautious about substituting one brand for another because they do have unique gelling properties. Pectin is available in both liquid and dry forms.
Kraft Foods makes two brands of fruit pectin including SURE-JELL, a dry product, and CERTO a liquid fruit pectin. These products are available in most grocery stores or buy online.