Source control is a mechanism where you store all RoboHelp project files in a central repository. Every author works with a local copy of the project connected to the central repository. Whenever an author wants to make a change, she checks out the document from the repository. This blocks other authors from making changes to the file until the author checks in her changes. Because the files are in a central repository, all changes an author makes are instantly available for all authors.
Source control is also a great disaster recovery option. Whenever a project is corrupted, the author can simply remove the local project and get the latest version of the project from the repository. And if the latest version in the repository is corrupted, it is very easy to rollback the repository to a previous version.
This post lists all the source control options for Adobe RoboHelp with hints on how to get them to work.