That is a tough one to answer, especially when you have to read the answer to get what you want to know and you don't understand when you read (basic question declares that). there are several possibilities, though. Some people are not reading learners, they are show and tell learners. I learn way better when someone talks to me than when I read the very same things, and I do understand what I read, generally very well. So that is likely part of it.
The second likely part is that you do not link what you read to that part of your brain that has to interpret. You haven't trained yourself to make that happen, and that is a learnable skill, something that you can improve by practice. read and think, read and think, read and think. Don't just read, then try to remember what the heck you just read and then understand it. It is a lot of work for some people to train themselves to do this. If you don't get what you just read, read it again, until you do. break it down into bits if that is what you must do. Once you get used to it, you won't generally have to keep stopping to do that. It is very slow work at the beginning. It can be very difficult for some people as well. We are not all the same, after all.