Product Backlog Refinement
Product Backlog items (PBI) evolve over time. They start as vague ideas and are improved as more information is uncovered. Eventually they are well enough defined that they can be brought onto a Sprint Backlog.
The activity of continually improving PBIs until they are ready to be worked on is called “refinement.” Most Scrum Teams refine during working sessions in which the Product Owner and Developers have focused discussions about the items in the Product Backlog. During these sessions they create a shared understanding of the goals for each item and discuss the order of the items in the backlog.
Refinement of the Product Backlog may happen many times during the Sprint. In some Sprints, the PBIs at the top of the backlog may be sufficiently refined and refinement may not be needed at all. Therefore, Product Backlog refinement is not a prescribed event in Scrum. However, it is good practice to have the Product Owner, Developers and stakeholders engage in these in-depth discussions about the items in the backlog on a regular basis. This creates greater clarity and a shared understanding. Refinement should happen as often as required.
Talking about Sizing and Forecasting in Scrum - an InfoQ article by PST John Coleman