This idiom or expression like sentence means:

Be careful.
You are under surveillance.

Also:
I’m watching you.
I’ll be watching you. Etc.

This idiom or expression means:

Watching someone carefully.

I’m not sure about this guy. Keep an eye on him for a while until we know more.

This idiom or expression means:

Supporting someone.
Watching someone’s back for support.

A. Hey Joe, I don’t know if I should do this?!
B. Go ahead, I have your back. I’ll help you if necessary.