A GameObject is Unity’s basic scene object. It is best understood as a named container in the Hierarchy. By itself it does very little; its behaviour comes from attached components such as Transform, SpriteRenderer, Rigidbody2D, Collider2D or scripts. (course lab series, see source-csharp-unity-labs)
For beginners, the important rule is: select the GameObject to inspect or configure the components that belong to it.
Related: component, monobehaviour, inspector, unity-transform