It usually means either: (1) Gotchya! You have been tricked. I have fooled you. For example, If I pretend I'm pregnant, then I say April Fools! I had you believing my prank for a minute! or (2) I had the game in my hands, I had you defeated, but I let it slip, and now I might not win this game. For example, when playing chess, if I am capable of winning but I miss my opportunity. Or, for example, in Jiu Jitsu sparring, if I can break your arm but I decide not to, I could say, I had you there! And I would hope you admit defeat, instead of pretending it was a draw.

I added an answer, but the italki computer deleted it, and banned me for 30 days. BUT italki got a REAL person to check and I am UNbanned again! So I will try again... You must be careful with the use of TO HAVE SOMEONE because it has another meaning. I cannot write the meaning because the computer will ban me again. It means ... something rather biological... about what you do... to reproduce. Oh dear! I hope the word 'reproduce' does not get me banned. So I cannot say what it means. But you maybe can guess. So if you say, "I had her there" it could mean several different things. CHECK the use, the context, and the situation. Be careful!

I think it could mean many things. If we were playing football and you put the ball through my legs you could say “I had you there!” Or If you were just joking but I believed you, you might laugh and say “I had you there!” Sorry if this doesn’t help, I’m not a language teacher 😂 I am English so I’m just imagining scenarios that I would say “I had you there!”

"To have" is one of the most important words. There is hardly a paragraph that fails to use this word many times. As such, it has hundreds of uses. "Have" has hundreds of synonyms. The possibilities for what "I have you" might mean are endless. Context prevails. "There" also has many possible meanings in your sentence. It can refer to a place, a time, or a situation. I have not, however, answered your question. What I have said so far only indicates that any answer that I give must necessarily be so vague as to be almost useless. The real but useless translation of the sentence is: "In some unspecified way, and in some unspecified place or time or situation, I possessed, controlled, mastered, included, or [...insert here any of hundreds of synonyms for 'have'] you." The examples in the comments are good, but are just starters for a long list.

"I had you there!" means "I tricked you" or "I made you believe something that wasn't true." It's often said after a successful joke or prank.