What do people mean by “hard to read”?

I've always found this statement quite weird honestly. Most of the time I can just grab a book and just read it without much difficulty. Even if the author's style is not that clear at first, if I keep reading it I can usually get used to his writing style.

Not to say I haven't found difficult books. Lord of the Rings was a challenge for me because of some of Tolkiens descriptions. Luckily, I found a vlog that explained perfectly all the parts I didn't manage to understand and that made it way easier (sadly, I can't remember its name).

Another hard author has been Shakespeare. I have found myself needing to reread some parts several times to try to make some sense of them and I'm not even sure if it's correct.

The hardest by far has been Cervantes with Don Quijote. The language is so outdated (read it in spanish) it feels almost impossible to understand without a guide.

Nietzsche has also been a very hard author but for very different reasons than the ones I've talked about.

Anyways, what do you consider is a difficult read? What's a challenge you encounter sometimes in your lectures? That's something I've been really curious about and would like to know.