Triggers are used to fire Actions and Conditions when something happens. This something can range from the player pressing a certain key to an NPC's entering a certain area.
(Example of a Trigger with the Input options open)
The white cog at the top right of the component there's an Advanced Options foldout. You can specify a minimum distance from which the Player has to be for this trigger to be activated.
Some times you'll want a Trigger to execute an Action only once. You can easily take advantage of an Action's
DestroyAfterFinishingproperty to destroy both the Action and the Trigger.
To do so, simply add an Actions component inside the same Game Object where the Trigger is. After executing the Actions, it will destroy the entire Game Object.