A tab is an attached flap or piece of material. You use a metal tab to open a can of soda, and you might use plastic tabs to separate sections in your binder.
Some tabs help you open packaging ("Grasp the plastic tab and pull to open"), and others give you information about or help you identify something. The noun tab also means "an additional document or page" on a computer screen, as well as "a space" on a keyboard or typewriter. In North America, it's another name for a bill as well: "Put it on my tab, please!"