An Equation Of Time (aka EOT) complication indicates the difference between "true" solar time (that of nature) and "mean" solar time (that of man).
As the earth orbits around the sun in an elliptical (oval) shape & the axis is tilted - there are only 4 days a year when the day is exactly 24 hours long April 15th, June 14th, September 1st and December 24th.
All other days of the year the days are shorter or longer - depending in the position of the earth. 
This watch will show the difference between the "mean" time & the "true" time. 
Since the of the days are fixed year after year (at the same location) a watch can be manufactured to replicate the correction via a shaped cam which elongates & shortens the days accordingly.
Decorated skeletonized rotor - can be personalized for an additional cost, the standard version has the AP letters.