A beep is a brief warning sound. You might give a quick beep on your car horn to let a bicyclist know you're passing her on the road.
Your friend's voice mail might say, "Leave a message after the beep!" and your smoke alarm might emit an annoying series of beeps when its battery is dying. In either case, you'll hear a high-pitched, brief tone. The word is imitative — it sounds like what it means — and relatively new, only as old as the car horns it imitates. Beep was first an interjection in the 1920s, and a noun and a verb by the end of the decade.