Items at the top of the backlog are ready to be moved into a sprint so that the development team can confidently commit and complete them by the end of a sprint.
A checklist of conditions that must be true before a product backlog item is considered ready to pull into a sprint during sprint planning. Contrast with definition of done.
Some Scrum teams formalize this idea by establishing a definition of ready. You can think of the definition of ready and the definition of done as two states of product backlog items during a sprint cycle:
DoR: Ready state from PB to Sprint Backlog