A trigger is a collider configured for overlap detection rather than physical blocking. In Unity 2D, a trigger overlap can call OnTriggerEnter2D(Collider2D other), which is commonly used for coins, hazards, checkpoints and exit zones. (course lab series, see source-csharp-unity-labs)
If a trigger does not fire, check for a missing Rigidbody2D, wrong Is Trigger setting, tag mismatch, disabled objects or layer-collision settings.
Related: unity-collider2d-and-triggers, collider2d, rigidbody2d, unity-gamemanager-pattern