Barriers are a temporary pool of additional health that takes damage before Health is reduced. Barriers block all incoming damage from enemy players, with Damage Resistances applied.
Any instance of excess damage dealt to completely destroy a barrier is then done to health.
For example, a Dynamo with 100 barrier health and 0% Spirit Resist takes 168 spirit damage from Vindicta's Assassinate. 100 spirit damage will break Dynamo's barrier, the remaining 68 spirit damage is done to health.