To back up means to move in reverse, going backward instead of forward. When a car backs up out of a driveway, the driver shifts into reverse and carefully moves the vehicle backward, often checking mirrors and looking over their shoulder to avoid obstacles.

The phrase also means to support someone or confirm what they're saying. If your friend tells the teacher about something that happened on the playground, you might back them up by agreeing and adding what you saw. When scientists make a discovery, other researchers try to back up the findings through their own experiments.

Back up can also mean making a copy of something important to keep it safe. When you back up the files on a computer, you create copies stored somewhere else. That way, if your computer breaks or gets lost, you haven't lost your work. People back up photos, documents, and other digital files they don't want to lose.

In all these meanings, there's a sense of support or protection: moving backward to get into position, supporting someone's story, or protecting your data by creating a safety copy.