Reading a book isn't a binary process. It's a gradient. Some books you fly though, not really paying attention, some you need to read, re-read, read discussions about it, think, discuss, etc. Some books I read and have no idea what's going on, but I let all the terminology wash over me and I pickup tidbits here and there. Then I read it again later after I've read some more stuff. Depends on how much I care. I read a LOT of really random stuff or I listen to lectures. I don't stress if I don't understand some concept. It's clear once you start working with the material.

Most people I've met have had reading ruined for them by school. Instead of thinking of reading as a fun, intellectual exercise they think of stress and boring. I heartily recommend How To Read a Book https://libgen.is/book/index.php?md5=81F5C5943EFCB5F70B7CABEEC89AFA41 if you want some help here. Really helped rehabilitate me after school even though I was reading for pleasure already.