Originally Posted by
Restingvoice
Rebirth's main theme is reintroducing old elements that were missing in New 52.
Hard to say without a spoiler, but the gist is New 52 Tim had a very different story and much-hated back story, so Detective Comics Rebirth is bringing his old and beloved back story back, and so he acts as if that's always been his back story, but he hasn't remembered everything that happened before New 52. He's still mostly Tim from New 52 with New 52 experience.
Jean-Paul Valley is also purely New 52 character, but he didn't feature much in New 52. His first appearance was in Batman and Robin Eternal, which is only about a year or half a year before Detective Rebirth.
The way Detective Rebirth is written though, you can just jump in. You just need to view them as new characters.