This command is meant to safely cancel the effect of `cvs checkout'. Since CVS doesn't lock files, it isn't strictly necessary to use this command. You can always simply delete your working directory, if you like; but you risk losing changes you may have forgotten, and you leave no trace in the CVS history file (see section The history file) that you've abandoned your checkout.
Use `cvs release' to avoid these problems. This command checks that no uncommitted changes are present; that you are executing it from immediately above a CVS working directory; and that the repository recorded for your files is the same as the repository defined in the module database.
If all these conditions are true, `cvs release' leaves a record of its execution (attesting to your intentionally abandoning your checkout) in the CVS history log.