Describe the main functionality of the application.
If more than one exists, then descripe tehm all but keep it at a maintainable level.
The main areas that the application acts on
Any special stuff regarding the application
The architecture and the main responsebility for the application
The database(s), tables, schematics that are relevant for this application.
Remember to keep this maintainable.
Diagrams could be usefull here.
Step by step guide on how to debug the application.
Location on those
Internal:
Eksternal:
Other stuff