MonoBehaviour is the Unity base class used by ordinary gameplay scripts. When a C# class inherits from MonoBehaviour, Unity can attach it to a GameObject as a component and call special methods such as Start, Update, FixedUpdate, OnTriggerEnter2D and OnCollisionEnter2D. (course lab series, see source-csharp-unity-labs)
The key beginner point is that Unity calls those lifecycle methods. Students write them; they do not call them manually.
Related: monobehaviour-lifecycle, gameobject, component, unity-input