Can someone tell me please what is the difference between 3 difficulty modes in the game?
More monsters? Or they make more damage? Something else?

And one more =)

How to click on item to hear its description?

Thank you!

i dont know about the difficulty modes (i guess monsters do more damage), but you can hear the item description if you place the cursor on the item and wait for a couple of seconds if i remember correctly.

Correct.

And to answer the other main part of the question: The only difference I've seen is the amount of damage you receive when Luther takes damage. Just be ready to stock up on healing items, use more strategy in fighting enemies, and to have to use healing or damage-reducing magic more often.

Definitely not only do you take more damage, but the enemies are a lot harder to kill. I accidentally switched to the hardest difficulty mid-game and found enemies that used to be easy to kill were now almost invulnerable.