A paragraph should ideally be a smooth, succinct experience that goes through a bit of exposition, illustrates an idea, sums up the point, and primes the reader for the next paragraph.
In practice, a writer can get too caught up in all the things they have to say and fail to organize it all into bits an ordinary human being would be able to digest. The end result is a huge run-on paragraph that makes it difficult to recall the original point of it if there was one in the first place. The reader’s eyes glaze over and all they see is a wall of text.
This afflicts all written media, but it is particularly infamous for its effect on Comic Books. One of the first things learned in comics is how to use dialogue bubbles effectively; a writer not allocating space carefully will end up covering their panel with a bunch of text and white space. Eventually, the reader will realize that they’re primarily looking at plain text rather than the vivid form of visual storytelling that comic books are famed for. TL;DR indeed.
At best, a Wall of Text is just a signal of really heavy exposition. At worst, they are a warning sign that the author is soapboxing about something.
Like a lot of tropes, though, even this isn’t necessarily a hopeless evil. There are occasions where a person will go on and on in real life, or perhaps is giving one huge, unbroken speech or rant, and for these cases, “walls of text” aren’t necessarily a terrible formatting idea; they can help visually reinforce what’s happening in the story and match the experience of the reader to the experience in-universe. Using it like this, though, still requires skill and finesse to avoid making just another negative example.
Speaking in Panels is often a way to evade this trope while recounting what happened.
If Speech-Bubbles Interruption are used to show it’s not being listened to, see Wall of Blather. If the text is literally written on a wall in-universe, it might be a Room Full of Crazy. See Read the Fine Print if these kinds of text actually contain very important information. Ominous Multiple Screens is sort-of the video equivalent. Compare Doorstopper, usually related to works that are literally nothing but words.