When they are deemed "Ready" this means that they are ready to be inserted in the scope of a Sprint. When they are not ready, this means that the appropriate level of detail (and peer review) was not yet achieved. They were not yet subject to "refinement" (previous terminology included the term "Product Backlog Grooming"). As a developer don't touch them.
When they are "Done" this means they are ready for the PO approval.
The definition of "Done" is an important part of Scrum (relates to transparency) and deserves a full section on the Scrum Official Guide. There could also be a Definition of Ready (but this is optional).
The INVEST model
Each and every user story shall possess the INVEST quality attributes. Refine an Epic into User Stories and before considering them "Ready" make sure that the INVEST model is applicable to each one of the user stories you are working on:- The INVEST acronym (for Scrum US): https://en.wikipedia.org/wiki/INVEST_(mnemonic)
- The original article from Bill Wake (author of the "Exploring Extreme Programming" XP123 blog) can be found here: http://xp123.com/articles/invest-in-good-stories-and-smart-tasks/
PS. The latter link has a variant definition for the SMART Acronym for requirements: SMART tasks.
