Triggers are special colliders used to detect when other colliders enter their geometry, this can be used to script events. For example, you can detect when a player character enters a room and use this in your script to trigger an event.
Triggers don't generate collisions.
Note
Triangle Mesh colliders cannot be triggers.
Triggers use dedicated trigger events that you can register for:
Trigger.TriggerEnter
Trigger.TriggerExit
To learn more about using triggers see tutorial How to use a trigger.