A pager is an electronic device that's used for simple communication between two people. Most pagers beep or vibrate when they receive a message.
Once cellphones became commonplace, fewer people used pagers, which are a much simpler communication tool. Most pagers can only receive messages, and many of them are only capable of displaying numbers. If you used a pager during their peak of popularity in the 1990s, you would probably have paged your friend the digits of your phone number; your friend would then have called you on the phone. Today, pagers are still used by emergency services for quick communication.