When you visit a file that is under version control, the mode line indicates the current status of the file: the name of the version control back end system, the locking state, and the version.
The locking state is displayed as a single character, which can be either `-' or `:'. `-' means the file is not locked or not modified by you. Once you lock the file, the state indicator changes to `:'. If the file is locked by someone else, that user's name appears after the version number.
For example, `RCS-1.3' means you are looking at RCS version 1.3, which is not locked. `RCS:1.3' means that you have locked the file, and possibly already changed it. `RCS:jim:1.3' means that the file is locked by jim.