Forms are rectangular frames in your PDF document which contain objects, for example an image or a placed PDF. The way in which you edit these forms and the objects inside them is different from regular objects in your PDF document. For example, you can select a regular object and move it to a random position on the page in your PDF document. Objects in forms can only be moved inside the form and thus, you can use the form to crop the object.
Object moved outside a regular rectangle.
Object moved within a form: the object is cropped.