Actually, this is a good question! Pectin is used as a setting agent in jams and jellies. It's basically a fiber that you'll naturally find in many fruits and some vegetables (though there are natural and synthetic forms); it's the sugary sticky stuff that joins plant cells together. So when you heat it with more sugar and acid (often lemon juice or vinegar), pectin causes a thickening that is characteristic to jellies and jams.
Actually, this is a good question! Pectin is used as a setting agent in jams and jellies. It's basically a fiber that you'll naturally find in many fruits and some vegetables (though there are natural and synthetic forms); it's the sugary sticky stuff that joins plant cells together. So when you heat it with more sugar and acid (often lemon juice or vinegar), pectin causes a thickening that is characteristic to jellies and jams.