It means that it should be hosted locally somewhere in our local network. So no cloud services or something, we don't want to put that kind of data on other people's servers.
All GitBooks are essentially just Git repositories, which can be self-hosted. If your organization requires it to be hosted on premises I suppose you have a self-hosted Git service like GitLab.
In that case, GitLab Pages plays well with GitBook using GitLab CI. Otherwise, you can set up some CI/CD pipeline that automatically builds your books for you.
We're a place where coders share, stay up-to-date and grow their careers.
We strive for transparency and don't collect excess data.