The header of a Unity Object drawer refers to the element located at the very top of the drawer that contains information and controls related to the target of the drawer.

In GameObject drawers this contains the name of the target GameObject and controls for things such as changing the active state of the GameObject.

In Component drawers this contains the type name of the target, the header toolbar and typically a control that allows controlling whether or not the drawer is unfolded in the inspector.

In asset-type drawers this contains the name and type of the target, the header toolbar and usually some header buttons.