Publiccode.yml is a metadata standard for repositories containing software developed or acquired by the Public Administration, aimed at making them easily discoverabile and thus reusable by other entities.
By including a publiccode.yml file in the root of a repository, and populating it with information about the software, technicians and civil servants can evaluate it. Automatic indexing tools can also be built, since the format is easily readable by both humans and machines.
Full documentation is available here.
A guide explaining how to add the publiccode.yml file to your project can be found here.