What is pectin?
Pectin is a gelling/thickening agent. You can generally buy it in powder or liquid form from stores. But what it is, is actually a complex form of carbohydrates called polysaccharides. It's found in the cells of certain fruits and plants. Wikipedia has more info on the chemical information as well as a list of common fruits and vegetables, and their level of pectin. Citrus peels has about 30% of pectin, which is a lot compared to, say, cherries, which has only about 0.4%.
So when one is making jam/jelly/preserves from a fruit that does not have or does not have enough pectin in it to thicken, one adds pectin.
More info on Pectin:
Pectin - Wikipedia