The origin and meaning of this peculiar grouping of words is uncertain. Some theorise that it is a koan, a statement devoid of meaning designed to bring the minds of those that hear it to a higher state of being. Others believe that its purpose is to generate confusion in the hardy souls who actually attempt to read instruction manuals, thus making it even more difficult to understand what the manual is about. Many think that it is simply a sign of incredible anal-retentiveness.
The only known phrase that is more confusing is "This is a lie".
A curiously similar and possibly related sentence is "This Page Intentionally Left Blank". Please see the Guide Entry on this phrase for the relevant details. http://www.h2g2.com/A162497
For the sake of your peace of mind (not to mention your sanity), avoid contemplating the meaning of this sentence at all costs.