Package repositories are folders with Packages and a packages.txt text file that lists the package available in that folder.
The folder need to be accessible by HTTP or HTTPS.
The first line Contains the name of the repository and its description. Repository names are usually one word and very short.
The packages are listed with one package per line. The full package file name and its short description are separated by a colon. A second colon can be added to include a long description.
By default Radria is setup with base repositories; to access your custom repository you need to change your Site Manager Configuration.
Check this article on Setting up new package repositories.