Buffers for entering/editing log messages for changes which are about to be committed are put into Log Edit mode.
Sometimes the log buffer contains default text when you enter it, typically the last log message entered. If it does, mark and point are set around the entire contents of the buffer so that it is easy to kill the contents of the buffer with C-w.
If you work by writing entries in the ChangeLog (see Change Log) and then commit the change under revision control, you can generate the Log Edit text from the ChangeLog using C-c C-a (log-edit-insert-changelog). This looks for entries for the file(s) concerned in the top entry in the ChangeLog and uses those paragraphs as the log text. This text is only inserted if the top entry was made under your user name on the current date. See Change Logs and VC, for the opposite way of working—generating ChangeLog entries from the revision control log.
In the Log Edit buffer, C-c C-f (M-x log-edit-show-files) shows the list of files to be committed in case you need to check that.
When you have finished editing the log message, type C-c C-c to exit the buffer and commit the change.