The Definition of Done is a formal description of the state of the Increment when it meets the quality standards required for the product. The purpose of the Definition of Done is to:

During the Sprint, it helps the Developers identify the work remaining for an Increment to be ready for release by the end of a Sprint (B), by providing a clear and shared understanding of what “Done” means for each Product Backlog item and the Increment as a whole.

It creates transparency into the state of the Increment when it is inspected at the Sprint Review ©, by ensuring that everyone involved in the product development has the same expectations and criteria for evaluating the value, usability, and quality of the Increment.

It guides the Developers when creating a forecast at the Sprint Planning (D), by enabling them to estimate how much work they can realistically complete within a Sprint, based on their capacity, skills, and Definition of Done.

References: : [Scrum Guide], The Definition of Done : [Scrum Guide], The Increment : [Scrum Guide], The Sprint Review : [Scrum Guide], The Sprint Planning