CVS is the source code control system that the XOOPS core team uses. A source code control system lets developers keep track of the different versions of all their files, and allows different developers to work on different files at the same time, and then merge all their changes into a single new version of the software they're making.
Module developers can use CVS also, through the dev.xoops.org site.
For detailed information about setting up access to CVS for yourself, check out this page from the wiki on dev.xoops.org:http://dev.xoops.org/modules/phpwiki/index.php/HowToUseTheCvs
For more information about source code control systems in general and how they're used, check out these pages:http://software.ericsink.com/scm/http://infohost.nmt.edu/tcc/help/tool/rcs/why.html