CVS is a source code management system, used also for version management of the OpenACS packages. "check in" refers to a commit operation in cvs, where one pushes usually a local version of a file to the common repository.
For the variable substitutions, see e.g. "S 4. Advice, tips, funky things" in the following tutorial:
http://www.linux.ie/articles/tutorials/cvs.php
