Screen is a window manager for non-graphic terminals.
It basically allows you to switch between several windows. This allows you to use several programs at the same time, such as a mailer, a newsreader, a webbrowser, an editor, and a shell. All of this on the same terminal - no more required. Using only one terminal is also nice as you can share the console windows on the same FreeBSD or Linux machine with others. And there is no need to remember which consoles are taken and which ones you still need to log out from.