What is the meaning of "I got you"?

What does

I got you

mean?

Does it mean

Ok, I understood what you said

or

Hey, I know that you did that?

If they are both correct, when should we use which?

The meaning depends on the context. In (at least American) slang, saying "I got you" means either "I get what you're saying" or "I've got your back".

In child games of tag, saying "I got you!" means that you caught someone.

We need some context to be absolutely sure of the phrase, but it often means: "I understand what you're saying." The "got" has to do with "getting" understanding of the other person's words. So saying you "got" someone is short for saying you received what they were trying to convey.

More literally, it could be used to mean "I have hold of you" or "I touched you", but I'm not sure that applies in this case, because you felt the need to ask.