The SATOR square is actually a very interesting subject, and there is no agreement as to its actual origin or meaning. All we know for sure is that it shows up in the archaeological record all across the western Roman Empire - from Britain to Italy - and in some locations in the east as well.
The translation "the sower Arepo holds the wheels with/by effort" seems to be the most common. SATOR is masculine nominative singular, TENET a third person active indicative verb, ROTAS feminine accusative plural, OPERA probably feminine ablative singular. The problem with this is that AREPO doesn't seem to be a Latin word at all: it doesn't occur anywhere else as any part of speech, or even as a proper name. The other problem is that it doesn't really seem to mean anything - at least, not anything that makes it worth chiseling into walls all over the Empire. So what gives?
Many believe that the square was adopted and used by Christians as a sign to one another, and this is the interpretation passed about most frequently. (I've got a brick tile with the square carved into it; the slip of paper that came with it claims that early Christians used it to make themselves known to one another without inviting persecution. Good marketing in the US these days, if not certainly true.) This stems from the fact that the letters of the square can be rearranged into a cruciform shape, thusly:
A PATERNOSTER O
These are the first words in the Lord's Prayer (in Latin, of course) with the excess As and Os standing for alpha and omega, as in the Christian "I am the alpha and omega, first and last" idea.
Popular as it may be, the Christian connection is neither proven nor accepted by all. Some arguments hold that the square is a Mithraic artifact; Mithraism's popularity with the Roman Army would explain the wide distribution and presence in Britain, where few other Romans went willingly.
More recently, the SATOR square figured in "The Book of The Sacred Magic of Abra-Melin The Mage," which was one of the texts of the Order of the Golden Dawn (an occult secret society founded around the turn of the 20th century.) I don't have any knowledge of the beliefs mentioned by LordOmar above, but the square's ancient provenance, mysterious nature, and clever arrangement have made it a favorite of many throughout the ages. Serious investigation of its actual origin seems to be stalled and is likely to remain so unless new evidence turns up, but it makes a very interesting subject for speculation!
Here's a list of references on the subject:
Ferguson, John, The Religions of the Roman Empire Cornell University Press, 1970.
Kovach, Edith, ``The Clearing House Column'.' Classical Outlook, December/January 85-86 issue.
Milburn, Robert, Early Christian Art and Architecture, University of California Press, 1988. (This is the source of the A PATER NOSTER O rearrangement.)
Moeller, Walter, The Mithraic Origin and Meanings of the ROTAS-SATOR Square. Leiden, Brill, 1973.