Triggers are colliders used to detect other colliders enter it, which you can use 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.
Triangle Mesh colliders cannot be triggers.
Create a trigger
Create a Colldier (box, sphere, etc.)
Check Is Trigger
Use a trigger
Triggers, instead of collision events, use dedicated trigger callbacks:
void OnTriggerEnter(Collider c)
void OnTriggerStay(Collider c)
void OnTriggerExit(Collider c)
To learn more about using triggers see tutorial How to use a trigger.