We prefer to have 'ready' items at the top of the Product Backlog before Sprint Planning, which is done through Product Backlog refinement. However, nothing stops our flow of Sprints, for example, we do not delay the Sprint because the
items are not ready. In such cases, the 'unready' items would be selected for the Sprint anyway and refined during the Sprint. 'Ready' items are those that are clear and small enough to fit into one Sprint.