Role has different meanings, depending on the context. In a general sense, a role is a player's responsibility in a group. These roles include tanks, dps, healers, and other support. In particular for Rift, Roles are saved builds of Souls.
Additional souls are available for PvP combat:
Roles are purchasable from trainers. These are sets of souls that you can activate at any time. When purchased, the role is empty and you must choose 3 souls. The amount of skill points you have from your level will be available to place into these souls. At every level you will gain at least 1 skill point in every role. You are able to have 4 separate roles. Each additional role that you buy costs more.
You can swap between each role with ease, but only out of combat.