Event

An Event is something that can run Actions when triggered. How or when an event is triggered is implied by the event name. Help markers (? in a circle) will explain when an event is triggered if it's not clear.
Events are often given a name prefixed with On, e.g. OnEnter, OnStart OnHit OnRespawn. You can tell something is an event by the yellow lightning icon and an edit button in the Inspector Window.