A screen saver is a moving image or pattern that appears on a computer monitor when the computer hasn't been used for a while. Originally, screen savers protected older monitors from a problem called “burn-in,” where an image left on the screen too long would get permanently etched into the display, like a ghost image you couldn't erase.

Today's screens don't have this problem as much, but screen savers stuck around because people enjoy them. They might show flying toasters, starfields, bouncing shapes, aquariums, or family photos sliding across the screen. When you move your mouse or touch the keyboard, the screen saver disappears and your regular desktop returns.

Many people also use screen savers for privacy, especially in offices or libraries. If you walk away from your computer, the screen saver can hide whatever you were working on from curious eyes. Some screen savers even require a password to turn off, which helps keep others from using your computer while you're gone.

The term has expanded beyond computers. People sometimes say their mind goes into “screen saver mode” when they're spacing out or daydreaming, watching random thoughts drift by without really focusing on anything important.