It gives a +6 circumstance bonus to all Hide, Listen, Move Silently, Search, Spot and Survival Checks
It gives a +6 Dodge bonus to Dodge Defence.
+20 circumstance bonus to movement for the terrain.
May use the Hide skill even if the terrain would not usually grant cover or concealment. Can also use the Hide skill even if he is observed.
It is explained in the book fairly clearly. There are no additional abilities for a +6 beyond what was gained at +5 and the obvious increases to the the skills and the dodge bonus.