Epilog is an option for graphics functions which gives a list of graphics primitives to be rendered after the main part of the graphics is rendered.
In three-dimensional graphics, two-dimensional graphics primitives can be specified by the Epilog option. The graphics primitives are rendered in a 0,1 coordinate system.
See the Mathematica book: Section 2.9.3.
See also: Prolog, AxesStyle, PlotStyle, DisplayFunction.
This adds a large red point to the plot after the Sin function has been plotted.
Evaluate the cell to see the graphic.