This is the final confirmation step before a box is deleted.
If the box is empty, deleting it just removes the box. If the box still has items in it, they will be permanently deleted along with the box — this page will warn you if that's the case.
If you want to keep the items instead, go back and use the box's edit page to move or orphan them first (see the Deleting a box help topic). Once you click Delete here, this cannot be undone.