What are server.key and server.crt?

When I try to run the devflow-party-data-client via "npm run dev" it first complains that it cannot find server.key. And if I create an empty file of said name, it complains that it cannot find server.cert. It is unclear what data is supposed to live in these files and where that data should come from: generated? sourced from somewhere? what format? None of the READMEs provide information either and they assume that "npm run dev" succeeds as is, which is not the case.