Actions are incredibly useful parts of Ember, they provide a very simple interface for communicating events and changes. They are triggered by the actions of users, they are called by templates upon the action of a user. See Ember Guide on Actions.
As applications evolve, you may find that you want to trigger an action programatically. There is a private ember class that makes this possible. EMBER.TARGETACTIONSUPPORT Is a mixin that can be applied to a controller / component which provides an interface similar to the way that the templates work.