The game talks about "defensive stance" but I have no idea what that is or how to use it.

EDIT: I finished the base game and was again greeted with tutorial messages. The messages said that some skills need to be put in the left ability bar (normally you have your basic melee, ranged and magic attacks here). So skills like "berserker" and "defensive stance" replace your normal attacks. I only played melee so I don't know what other special skills there are.