Product Backlog Refinement (PBR) is the act of breaking down, adding details (such as: description, order and size) and further defining Product Backlog items into smaller more clearer items.
Product Backlog Items may be added, deleted and reordered.
The Product Backlog Refinement is a joint effort between the Product Owner and the Developers. The Product Owner is Accountable for the PBR.
PBR is not a prescribed scrum event, rather an ongoing activity.
PBR used to be called Product Backlog Grooming in older Scrum Guides.