What does your IQ score actually mean?

So far, I've scored around 133-138 on whatever IQ tests I found online, even 138 on Mensa. Now, I know that online tests aren't always accurate, but when people score lower than me in the same tests, that's how I know that my IQ is above average. Despite all that, why do I feel like I'm the dumbest person in the world, everyone knows more than me and if no one can do it, I can't do it either despite having more IQ than most? Is IQ limited to only mathematical abilities or something? Or maybe not even that, judging by the fact that I feel like I can't solve a math problem that not even the toppers can solve(I study Physics in university, for what it's worth). I don't know if there's any truth to it, like I said, I just feel like it. And other than that, I'm good at absolutely nothing. Not chess, not riddles, puzzles, nothing. I'm socially awkward too, to put salt in my wounds. So, what does IQ really designate? What does it mean?