What does keeping an eye on you mean as a idiom?

Keeping an eye on someone means you are watching them. You could be watching them for their own safety, or you may find them suspicious.

Example: "I wanted to go shopping, but Mom has me busy keeping an eye on my 2-year-old brother instead."

Example: "Even though it was Halloween, I decided to keep an eye on the shopper with the ski mask, just in case they were a real robber trying to blend in with the people in costume."