Sub is a shortened form of several different words, and which one people mean depends on the context.

1. A substitute, especially a substitute teacher. When your regular teacher is sick or away, a sub comes in to lead the class for the day. Students might also sub for each other on sports teams, stepping in when a teammate needs a break.
2. A submarine, which is a vessel that can travel underwater. Nuclear subs can stay submerged for months at a time, while research subs explore the ocean floor. The word works as a verb too: when a submarine subs, it dives beneath the surface.
3. A submarine sandwich, that long sandwich on a roll filled with meats, cheese, and vegetables. It got its name because the bread's shape resembles a submarine. Different regions call it different things: some say hoagie, some say hero, some say grinder.