Collider2D is the family of Unity 2D components that define collision shapes, such as BoxCollider2D, CircleCollider2D and PolygonCollider2D. With Is Trigger off, the collider can create solid contact. With Is Trigger on, it detects overlap without pushing objects apart. (course lab series, see source-csharp-unity-labs)
For beginner debugging, check whether the object has the right collider, whether Is Trigger is set correctly, and whether a moving object has a Rigidbody2D.
Related: unity-collider2d-and-triggers, trigger, rigidbody2d, component