What is the purpose of "Definition of Ready"?

Definition of Ready (DoR) is not a part of the Scrum Guide.   Many however use such a checklist to support the readiness of an item before it is accepted by a Development Team for a sprint.

The acronym INVEST is a popular item in many DoR's.   It really is up to each organization/team to determine the criteria an item should satisfy in order to reduce risk and increase the likelihood of completion.

As a minimum, an item should be estimated by the people who will be doing the work, and thought to be small enough to be completed by them within a Sprint.

Definition of Ready (DoR) is not a part of the Scrum Guide. 

Well, technically, the Scrum Guide contains a DoR, though not as an artfact:

Product Backlog items that can be "Done" by the Development Team within one Sprint are deemed "Ready" for selection in a Sprint Planning.

So the Guide defines ready as "You can do this in a sprint":  That certainly is a minimal Definition of Ready. If a team is using User Stories as PBIs, there's an additional Definition of Ready, as Stories should adhere to the INVEST criteria (Independent, Negotiable, Valuable, Estimable, Small, Testable).

However, teams should beware of making exhaustive checklists as DoRs because that may lead to a waterfall-esque situation.

Thanks guys for your advice! In a large organization, should we emplore a same DoR in multiple teams when they are working on the same product? I believe that would be eaiser for teams to do the estimation and select work in sprint plan meeting.

When multiple teams are working on the same product there is an additional consideration, which is that dependencies will need to be minimized for work to be Ready.

A joint refinement activity may be conducted on the understanding that certain teams are likely to work on certain items, and this will inform and shape not only dependency resolution, but the state of readiness which should best apply.