The Default Focus creates a short cut to accessing an Actor in your Blueprints and Code by setting the focus of the AI Controller. By setting the focus of the AI Controller to an Actor, you can directly access it from the AI Controller instead of accessing a Blackboard key.
This takes in an Actor Key (an Object Key with its base class set to Actor) to be quickly referenced by the AI Controller using
The name the node should display in the Behavior Tree graph.
As of 4.4 the Blackboard Key will appear to accept Vector Blackboard Keys, these will do nothing as the Set/Clear/Get Focus Actor functions operate on an Actor.