Definition of the Product Backlog
The Product Backlog is a dynamic, ordered list of requirements needed to improve your product. It is the sole source of work for the Scrum Team.
The work on the backlog is described in Product Backlog items (PBIs).
The Scrum Team refines and details these items until they are small enough to be completed within a single Sprint. These items are then ready for selection in the Sprint Planning.
Refinement
Refining the Product Backlog is also known as refinement. 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. It is actually an ongoing process and includes adding details, such as descriptions, prioritization and estimates.
Estimating the size of a backlog item, by the way, is 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.
A Scrum Artifact
Scrum recognizes three ‘artifacts’ representing work or value. They are intended to convey key information as transparently as possible. The Product Backlog is one of these artifacts. The other two artifacts are the Sprint Backlog and the Increment.