In the old days, candles were mostly made from tallow, a form of animal fat that provided an inexpensive way for people to light their homes at night.
Tallow is rendered animal fat, most often from beef or mutton. It's similar to suet or lard, other forms of animal fat, and has been useful in various ways through the years because it doesn't go bad at room temperature, stays solid, and is cheap. Today it's most commonly used for soap making or animal feed, though it can also be used in making biodiesel fuel.