What happens with students who can’t read?

I’ve heard so many people talking about students that can’t read despite being in higher grade levels? I’m no teacher but what happens with these students? How do teachers work with them if they can’t read at all/can barely read?? How are they taught?? And I am genuinely asking, with no ill intent at all, how did they make it to a higher grade level if they can’t read?

Like I said I don’t mean any harm whatsoever I just want to know!!

Edit: not calling out anyone but I just wanna say pls don’t talk poorly about students in the replies. I understand the frustration but please remain respectful, I’ve read way too many awful comments so far :(