if you need to delete Price Groups (if they are no longer relevant - or if you had made a mistake) you need to know that you are not going to cause any harm by accident. You don't want to accidentally delete PGs that are being used by your products.
For this reason, BRM shows an impact analysis if / when you delete a Price Group
Clicking on any of the links in that popup will take you to the Product Family in question so that you can see where the Price Group is being used.
Only delete PG's that are really not being used otherwise BRM will not know how to price up your Items (online or back office)