CVS means Concurrent Version System; it is a version control
system which can record the history of your files (usually,
but not always, source code). CVS only stores the differences
between versions, instead of every version of every file
you've ever created. CVS also keeps a log of who, when and
why changes occurred, among other aspects.

CVS is very helpful for managing releases and controlling
the concurrent editing of source files among multiple
authors. Instead of providing version control for a
collection of files in a single directory, CVS provides
version control for a hierarchical collection of
directories consisting of revision controlled files.

These directories and files can then be combined together
to form a software release.

Install the cvs package if you need to use a version
control system.