Add this attribute to a script class to mark its GameObject as a selection base object for Scene View picking.
In the Unity Scene View, when clicking to select objects, Unity will try to figure out the best object to select for you. If you click on an object that is part of a prefab, the root of the prefab is selected, because a prefab root is treated as a selection base. You can make other objects be treated as selection base too. You need to create a script class with the SelectionBase attribute, and then you need to add that script to the GameObject.