A giant is someone that's unusually enormous. You can also use the word giant to describe something or someone that is really big. That giant green guy over there? That's the jolly green giant. Ho ho ho.
A star basketball player might be described as a giant, or a stunningly tall redwood tree, or even a very large business. These giants come from the giants of mythology, huge creatures that are featured in fairy and folk tales and inspired by the giants of Greek myth, who were a separate race of strong, aggressive gods. The Greek root is Gigas, with the plural form Gigantes.