UI is user-interface, refer to the images in the quote post to get an idea. But basically, it's the black borders during normal gameplay and sex scene that was added in the game (Didnt touch the menu save/item screen). Most players hate it, as it blocks the already small screen and whatever other reasons. Regardless, it prompt a reason for someone to create a cleaner version.
Cleaning it, is personal preference. Either you like the new UI or you don't.
Hence, it is provided as an option. Hope that 'clears' it up