One thing about the ranks...

What the ranks are for, because I can't set for example that only members with this rank can view this forum for example.

ranks or for what ever you want to make them as.

like say you have a good poster you can set a rank for them called

Grand poster
or
Newbie
Advanced member
Moderator in training ?

idk what ever you want to create and call different ranks it's up to you

are the groups then the thing you limit the forums you can see?

yes you can set permissions for groups.
Ranks are just a title and or image you can set as a rank.. nothing more.