What's "back up" mean here? Isn't it like "support?" as in I need a backup?

Native speaker’s answer
Rebecca
That's right! Back up" as in "support" is one meaning of "back up!" Here, though, "back up" means to go backward in the conversation to a specific point or to provide more context to what is being said. Ex: Can you back up a bit? When did Lucy say she was getting a haircut? Ex: Let me back up. We were fighting because I didn't know the directions to the hotel. Ex: Wait, back up. I don't understand. Can you explain that a bit more?