Refinement
The ongoing activity of adding detail, estimates, and order to items in the Product Backlog, an act that occurs throughout the Sprint, not a formal event.
What it means
Refinement (previously called grooming) is an ongoing activity, not one of the five official Scrum events, though the Scrum Guide explicitly requires that it happen. It usually consumes no more than about ten percent of the Developers' capacity.
The Product Owner may influence or delegate refinement, but the Developers who will do the work are responsible for the final estimates and clarity, since they best understand what it takes to deliver it.
Common mistakes
Well-refined items are smaller, clearer, and have enough detail to be selected confidently in an upcoming Sprint Planning, reducing surprises mid-Sprint.
Example
The team holds a one-hour Refinement session midweek to break a large epic into three sprint-ready stories.