When you are finished editing the file, type C-x C-q again. When used on a file that is checked out, this command checks the file in. But check-in does not start immediately; first, you must enter the log entry---a description of the changes in the new version. C-x C-q pops up a buffer for you to enter this in. When you are finished typing in the log entry, type C-c C-c to terminate it; this is when actual check-in takes place. See section Log Entries.
With RCS and SCCS, a checked-out file is also locked, which means it is writable for you, but not for anyone else. As long as you own the lock on the file, nobody else can modify it, and nobody can check in any changes to that particular version. Checking in your changes unlocks the file, so that other users can lock it and modify it.
CVS, on the contrary, doesn't have a concept of locking. The working files are always modifiable, allowing concurrent development, with possible conflicts being resolved at check-in time. See section Using VC with CVS.
To specify the version number for the new version, type C-u C-x C-q to check in a file. Then Emacs asks you for the new version number in the minibuffer. This can be used to create a new branch of the file (see section Multiple Branches of a File), or to increment the file's major version number.
It is not impossible to lock a file that someone else has locked. If you try to check out a file that is locked, C-x C-q asks you whether you want to "steal the lock." If you say yes, the file becomes locked by you, but a message is sent to the person who had formerly locked the file, to inform him of what has happened. The mode line indicates that a file is locked by someone else by displaying the login name of that person, before the version number.