Provide enough information for people to have context for your changes. What problems the code is solving now, why have we picked this solution historically, etc.
Provide a short summary of the changes. Try to include answers to the Why? questions, e.g. Why are we introducing this change?, Why now?, etc.
If your changes make UI changes, consider providing before and after screenshots. You can also include a video demonstrating the old vs. the new UX.
Explain why things are done the way they are in this PR. Highlight the most important and/or controversial design decisions you have taken. Why it's implemented the way it is? What alternative implementations have you considered and why you chose this one?
The description is a good place to include questions that came up during development like Is this name the best one?, Could approach B be more applicable in this case?, etc.
This is also a good place to talk about performance and security considerations if any.