Here are some commonly asked questions about this topic.
The definition of done is a list of what must be true to consider a PBI done. The whole team creates and agrees to what is in the definition of done and is updated as needed for the team to function effectively.
Learn to use the definition of done and explore acceptance criteria vs definition of done.
It is the next complete piece added to build the product. The increment is complete in the sense that it should be ready to release to the end-user even if the team chooses to wait.
Learn more about incremental and iterative development or explore the essential Scrum glossary.