To complete an item from the Product Backlog or an Product Increment it is important that there is a common understanding of what “Done” means within the project team.
Only when you defined what “Done” means, you can find out if work on an item or on the whole Product Increment is complete or not.
The definition od “Done” guides the Development Team in understanding how many items of the Product Backlog they can accomplish during a Sprint.
For achieving a high quality it is useful that the definitions of “Done” contain stringent criteria.