A prong on a fork is the pointed part used to spear food, according to Dictionary.com. It is also called a tine. As noted by Etiquette Scholar, the number, size and shape of tines depends upon the type of fork.
A typical dinner fork has long tapered prongs. A lobster fork often has one long tine in the center flanked by two hooked prongs, while a general seafood fork has three short prongs. To spear fruit easily, a fruit fork has narrow tines. Salad forks tend to have flat, broad prongs. On both salad and dessert forks, the left tine is wider than the rest. This improves the diner's ability to cut with a fork.