Prolog is an option for graphics functions which gives a list of graphics primitives to be rendered before the main part of the graphics is rendered.
Graphics primitives specified by Prolog are rendered after axes, boxes and frames are rendered.
In three-dimensional graphics, two-dimensional graphics primitives can be specified by the Prolog option. The graphics primitives are rendered in a 0,1 coordinate system.
See The Mathematica Book: Section 2.9.3.
See also: Background, DefaultColor, Epilog, AxesStyle, PlotStyle, DisplayFunction.