The JEDI Version Control System is a source-code-control or version-control system much like SourceSafe. It is possible to work with remote repositories over the internet. It has many advantages over first-generation Windows version control systems (like SourceSafe), but given the more powerful version control systems like Subversion, and the windows "TortoiseSVN" client, are much more popular than the JEDI VCS, it is unlikely that the JEDI VCS will see much use even among Delphi developers.
Unlike Subversion, JEDI VCS offers full IDE integration, and was developed especially by Delphi developers for Delphi developers, thus there are those who prefer this version control system to others. Because it's completely free, you can try it for yourself without risk.
The two most active JEDI projects (the JCL and JVCL) do not use the JEDI VCS for their own ongoing development, rather they use Subversion repositories, and most of the JEDI JCL and JVCL developers use TortoiseSVN intead of the Jedi VCS.