Creating directories

There is an example of a recipe which I am trying to copy in
procmailex.0. It shows how to create a $MONTHFOLDER directory
if one doesn't already exist. I'm trying to do a similar thing
with mailing lists, but I can't get it to work, or even
understand what is supposed to happen.

Can someone explain how this recipe works? :-

     MONTHFOLDER=`date +%y-%m`

     :0 Wic
     * ? test ! -d $MONTHFOLDER
     | mkdir $MONTHFOLDER

     :0:
     * meeting
     ${MONTHFOLDER}/meeting


Please bare in mind I am very new to procmail, and I'm running
under OS/2 some certain shell specific functions may not apply.

Thanks,

--
John