Definition of Product Backlog Refinement
Product Backlog Refinement is the process of detailing the Product Backlog by the Scrum Team. Detailing means adding details such as descriptions, prioritizations and estimates to Product Backlog Items (PBIs).
The purpose of refinement is to describe valuable Product Backlog items in such a way that they can be completed within a single Sprint. These PBIs are then ready for selection during Sprint Planning.
The estimation of the size of a backlog item is always the responsibility of the Developers. The Product Owner may influence the Developers by helping them understand the trade-offs and making choices during this process.
In previous versions of the Scrum framework, it was recommended that refinement should not take up more than 10% of the Developers’ available time. Nowadays, there is no set limit and refinement is actually an ongoing process during the Sprint. Note: It is not an official Scrum event as defined in the Scrum Guide.