The foot is the end you light. On most cigars it is cut clean and left open, so you can see the filler, binder, and wrapper in cross-section. It is at the opposite end from the head and its cap.
Lighting well mostly depends on the foot. Toast it evenly before you draw, keep the flame just off the leaf, and turn the cigar so the whole end lights. A torch and a soft flame both work. Some cigars have a specially finished foot: a closed foot on a perfecto, a narrowed foot on other figurados, or a “shaggy” foot, where the filler extends past the wrapper and is left untrimmed. These finishes change the first few puffs, not the blend.
A cracked or dried-out foot is a sign of poor storage. See humidor and relative humidity. For step-by-step lighting instructions, see how to cut and light a cigar.