Oh, I understand now. What happens is at certain levels you will unlock being able to use skillbooks (I've gotten a few in dungeons), that can be used to upgrade an Ultimate. Say for example the passive that increases DMG +5%. At a certain level, you will unlock being able to use it. At a higher level, you will unlock the upgrade for it. The Ultimate that increases the 5% to 7% will be blocked off until you use the book.
Take a look at the blocked off Ultimates in each section. Some of them have no level requirement while some of them have a requirement. The ones with no requirement means you have reached the level you can use the book.