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 and we do not delay the Sprint because the items are
not ready. In such cases, the 'unready' items would be selected for the Sprint and refined during the Sprint. 'Ready' items are those that are clear and small enough to fit into one Sprint.