Product Backlog Refinement is the collaborative process in a team for adding detail, estimates, and order to the items in the Product Backlog.
In this session, the items are reviewed and revised by Product Owner and team members, by gathering information about the backlog items. This helps to develop a clear and common understanding about the requirement among the team members.
Refinement session gets the Product Backlog item to “Ready state”. Teams consume their 10% iteration capacity for refinement. This gives a chance for the team to ask questions and get it clarified, and get acquainted with backlog items before the Sprint planning. Meanwhile, the Product Owner’s get more time to in clarifying queries, giving enough information and confidence for team to accept prioritized items. Hence, the Product backlog refining helps the sprint planning to go smooth.
Further Reading:-
https://www.scrumalliance.org/community/articles/2014/october/product-backlog-refinement