CVS organizes information in modules. A module is generally a directory in the source tree. OpenBSD is somewhere in between, with smaller modules for large chunks of the operating system. This file lists every module you can check out of that repository. This is convenient if you want the latest version of a port, but don't want to upgrade your whole source tree or bother to FTP out for it.
Some modules contain other modules.
You can grab the whole usr.sbin tree, or just one part, with proper selection of module names. CVS also allows you to access arbitrary chunks of a project's source tree. All you need is the path to the portion of the source tree you want. For example, various developers are working on integrating portions of the BSD make system. You can specify revisions with the -r flag.