Sources were a highly requested feature for Paperback as it allows users to read manga that are otherwise not on MangaDex (The default source of the app). The sources are community-driven, and may be updated and changed without requiring an application update. This allows for a rolling development process, implementing changes as they come in, rather than waiting for the application developer to implement new sources and bugfixes.
To find out more about developing a source, check out the extensions common repo wiki, use one of the current implementations in the repo as an example, and/or be sure to ask in the Discord!!