What problem are you trying to solve? If there is a PRD, this can be a sync’ed block from the PRD. Link to any other documents that are relevant for background or context.
What should be true after this project is implemented?
What is explicitly not in scope and why?
What changes are required to solve this problem and achieve the project goals?
What are the high-level implemented changes?
Diagrams can be very helpful here.
What are the high-level historical changes?
What are the main changes to the main source of the problem?