GNU Emacs Manual
M.7.2 Version Control and the Mode Line
When you visit a file that is under version control, Emacs indicates
this on the mode line. For example, `RCS-1.3' says that RCS is
used for that file, and the current version is 1.3.
The character between the back-end name and the version number
indicates the version control status of the file. `-' means that
the work file is not locked (if locking is in use), or not modified (if
locking is not in use). `:' indicates that the file is locked, or
that it is modified. If the file is locked by some other user (for
instance, `jim'), that is displayed as `RCS:jim:1.3'.