By default this is on and allows you to paint on the paintable objects. If you want to paint on the grid plane in Perspective view instead, then select Paint on View Plane (see Paint Effects > Paint on View Plane). These two menu items - Paint on Paintable Objects and Paint on View Plane - are toggles; when one is selected, the other is turned off.
If this is on (check mark beside the menu item) and there are no paintable objects in the scene, your strokes will lie on the grid plane of the perspective view.
