A cub is a baby animal. A mother fox sometimes carries her cub by the scruff of its neck.
Use the word cub when you talk about one of a number of meat-eating mammal babies, including bears, foxes, lions, and tigers. While a tiny bear cub looks as adorable and helpless as a stuffed animal, its mother is ferocious and protective of her cubs. In the old days, people also called a young, immature boy a cub — today, a human cub is more likely to be a junior Boy Scout — a Cub Scout.