A screen saver is a software program that becomes activated after the computer has been inactive for a specified amount of time. Screensavers were originally designed to help prevent images or text from being burned into older monitors. The monitors and LCDs used today no longer suffer from this issue, but screensavers are still used for their aesthetic value, and to password-protect the computer when the user is AFK.
Tip: In Windows press the Windows key + L to lock the computer if you're stepping away from the computer. Locking the computer allow the screensaver to run, but prevents someone from accessing your computer.