When you delete categories… if the category is associated with a product as the default category… then the first category in the list will become the new default category (see Error! Reference source not found..) Once the category is deleted, it cannot be recovered. 

Final Warning: Deleting the selected categories will also delete any sub-categories for each of the selected categories. If you proceed to delete the selected categories, any products that are listed in these categories will not deleted, rather they no longer will be listed in these categories since these categories will no longer exist, and those products will continue to display in other categories the product is assigned to. Are you sure you wish to delete all the selected categories and their associated sub-categories.

v  OK:  Will complete the delete operation.

v  Cancel:  Terminates the deletion process.