"Patch welcome" is a colloquial term to indicate that at least an initial stab at a solution to a ticket would be useful to have developers review the ticket.
For unscheduled tickets, where many "patch welcome" type tickets end up, the requirements are a bit stronger: as no Trac developer has shown interest in making the bug or feature request part of the Trac roadmap, the patch has to be very convincing:
- well exposed rationale for the change
- clarity and correctness of the code
- unit tests, functional tests
See TracDev/SubmittingPatches#Whatisagoodpatch for more details.