UnityEngine.Object is the base class for all built-in Unity objects. Don't derive from or instantiate this class directly. Instead, use the appropriate subclass such as GameObject, MonoBehaviour, or ScriptableObject.
Any public variable you make that derives from UnityEngine.Object is shown in the Inspector as a drop target, allowing you to set the value from the GUI.
Some APIs are designed to work with any Unity Object, so Object appears as a type in their signatures. For example, Resources.LoadAll, EditorJsonUtility.ToJson and SerializedObject.
Each instance of a class that derives from UnityEngine.Object is linked to a counterpart native object. If the native counterpart is destroyed before the managed object is garbage collected, or if the instance references a missing asset or missing type, the managed instance can be in a detached state.
Detached objects retain their InstanceID, but the object can't be used to call methods or access properties. Comparing objects in this state with null evaluates true, because of Unity's custom implementation of the equality (==) and inequality (!=) operators and Object.bool. However, because the managed object is not truly null, a call to Object.ReferenceEquals(myobject, null) returns false.
The null-conditional operator (?.)
and the null-coalescing operator (??) are not supported with Unity Objects because
they can't be overridden to treat detached objects the same as null. It's only safe to use those operators if the checked objects are guaranteed to never be in a detached state.
Additional resources: Object in the Unity manual.