Non-nil means make a backup of a file the first time it is saved.
This can be done by renaming the file or by copying.
Renaming means that Emacs renames the existing file so that it is a
backup file, then writes the buffer into a new file. Any other names
that the old file had will now refer to the backup file. The new file
is owned by you and its group is defaulted.
Copying means that Emacs copies the existing file into the backup
file, then writes the buffer on top of the existing file. Any other
names that the old file had will now refer to the new (edited) file.
The file's owner and group are unchanged.
The choice of renaming or copying is controlled by the variables
`backup-by-copying-when-privileged-mismatch'. See also `backup-inhibited'.