A concise description of the changes (bug or feature) and their impact/motivation.
If this description is short enough to be used as the title, delete this section and just use the title.
For bug fixes, also include specifics of how to reproduce it / confirm it is fixed.
List any specific things you want to get reviewer opinions on, and anything a reviewer would need to know to review this PR effectively.
Things you might want to include:
Questions about how to properly make automated tests for your changes.
Questions about design choices you made.
Descriptions of how to manually test the changes (and how much of that you have done).
etc.
If you have any questions in this section, consider making the PR a draft until all questions have been resolved.
Description
Reviewer Guidance
The review process is outlined on this wiki page.