This acrostic is the famous sator formula. It can be translated as:
 
 
The text may be read in four different ways:

(i) horizontally, from left to right (downward) and from right to left (upward);

(ii) vertically, downward (left to right) and upward (right to left).

The resulting phrase is always the same.
 
It has been suggested that it might be a form of secret message.
 
This acrostic was unearthed during archeological excavation work at Pompeii, which, as is well known, was buried by the eruption of Vesuvius in 79| A.D.  The formula can be found throughout the Roman Empire, probably also spread by legionnaires. It has, moreover, been found in Mesopotamia, Egypt, Cappadocia, Britain and Hungary.

The sator acrostic may have a mystical significance and might have been used as a means for persecuted Christians to recognize  each other (it can be rearranged into the form of a cross, with the opening words of the Lord's prayer, A Paternoster O, going both vertically and horizontally, intersecting at the letter N, the latin letters A and O corresponding to the Greek letters Alpha and Omega, beginning and end of all things).