If check in comments are neither required nor optional, then the selected documents are checked in, and you are done.
or
If check in comments are optional or required, the Check In Document(s) dialog opens. Enter change comments (or select from the list of previously entered comments) and click OK.
Note: If comments are optional, you can click OK without entering comments and the document will still be checked in. If comments are required and you click OK without entering comments, you get a notification that a comment must be entered to complete the operation.