There are a number of ways this can be used, with different meanings depending on the context. One common use is to say that I nearly succeeded in tricking you. For example: for a joke, I pretend that I lost the book you lent me. If you believe me and get upset, then I 'had you'. If you're not sure for a moment, and then realise that I am joking then I 'almost had you'. If you realised straight away that I was joking then you are clever, and I didn't even get close!