This command is part of the posix extensions and needs to be imported to be available to the shell.
See Posix Commands
|-r,-reference reference_file||Use reference_file as the source for the timestamp|
If file does not exist, creates a new empty file. Otherwise updates the last modification time of the specified file.
If "-r reference_file" is given, then instead of setting the last mod date to the current time, sets it to the same last mod date of the reference file.