A component is a modular piece of data or behaviour attached to a GameObject. A Transform component stores position, rotation and scale. A Collider2D defines a collision shape. A script that inherits from MonoBehaviour is also a component. (course lab series, see source-csharp-unity-labs)
Unity’s component model means students should ask two questions when debugging: which GameObject is selected, and which component on that GameObject owns the setting or behaviour?
Related: gameobject, monobehaviour, getcomponent, inspector