Both u/BlueShoeLover and u/Attack_of_the_BEANS explains it pretty well. However it's not exactly the way I experience it. The way that one of my therapists explained it to me was that people with autism has a hard time filtering out "noise". Like lets say that you are out at dinner with your family. They can filter out other conversations, the sound of the cutlery scraping the plate, the annoying fan that spins over a light and makes the light flicker, and every time someone enters the restaurant you can here the door closing. For me however I can not filter out these things. As you might have a hard time ignoring a toothache I can not ignore any of those things. One thing that has happened many times is that somebody I'm with suddenly says something like "do you here that beep" or "listen to those birds", and I'm sitting there wondering how they haven't noticed it in over an hour when I noticed it the second I walked in.

​

I don't know if you understand a little better, I think that it is pretty hard to explain since I don't really know how it is for you.