Each attuned soul grants players access to a unique "Soul Tree," which contains all of the traits and abilities that a soul had mastered during its life. It is called a tree due to the branching structure of the skill progression. As your character levels up, you earn soul points that can be used to improve that tree's soul level, granting players access to even more powerful traits and abilities
Players invest their soul points into the branches of the Soul Tree, picking and choosing the specific abilities and passive traits they desire. With each point spent, the Soul Tree increases in level, periodically unlocking new advancement options higher up the tree.
Players automatically unlock new "root" abilities as the Soul Tree increases in level. If the branches represent the unique choices a player makes, the root represents the core power of the soul that all players have access to. Root abilities unlock quickly at first, but slow down as the tree’s soul-level increases. 
- Official Rift Game page on the class system - Riftgame.com