Clarifying is a technique for removing impurities from a liquid, usually a stock, broth, or sauce, so it turns clear rather than cloudy. The liquid is heated gently and any foam, fat, or fine particles that rise to the surface are skimmed off, sometimes over multiple simmerings.
For very clear results, such as a classic consommé, cooks add a "raft" of egg white or ground meat to the liquid before it simmers; the proteins coagulate and trap fine particles, and the raft is removed along with them.
Clarification is central to classic French cuisine (consommés, aspics) but the same idea shows up elsewhere - clarified broths underpin Chinese soups and hot pot, and dashi is sometimes clarified in Japanese cooking. It's time-consuming but produces a cleaner flavor and a transparent, polished appearance.