If work items are not precisely understood, development effort (and time) tend to balloon, which in turn cause the Sprint to fail. Therefore, define criteria which items must meet before they can be put on the Sprint backlog. This done criteria is often called “ready” as in “is this story ready for a Sprint.”

Most call this definition the "Definition of Ready (DoR)?". This is related to Team Has Right to Reject Stories for Estimation That are Not Ready