A leg is a weight-bearing and locomotion device. Birds and humans have two legs, but many animals have four or six or even more. A biped is an animal with two legs and quadruped is an animal with four legs. Centipedes and millipedes have many more legs.
By extension, the word is also used for some objects. For example tables and chairs, also have legs to hold them up.
People also use the word "leg" in idioms, for example: