PostScript["string"] is a graphics primitive which gives PostScript code to include verbatim in graphics output.
Mathematica by default renders a point with coordinates 0 0 in the PostScript code at the bottom left-hand corner of your plot, and a point with coordinates 1 r at the top right-hand corner, where r is the aspect ratio of the whole plot.
You can specify a bounding box for the objects represented by your PostScript code by including a standard conforming PostScript comment of the form %%BoundingBox pxmin pymin pxmax pymax.
PostScript["string", xmin, ymin, xmax, ymax] then renders the point with coordinates pxmin pymin in the PostScript code at position xmin, ymin in the Mathematica graphic, and the point with coordinates pxmax pymax at position xmax, ymax.
Mathematica will transform graphics represented by a PostScript command to make it fill the specified rectangle.
After execution of PostScript code included by the PostScript command, all PostScript stacks must be restored to their original states.
The utility of the PostScript command depends on your PostScript interpreter's ability to process the PostScript commands you specify.
Display may or may not convert graphics produced by PostScript commands to other formats.
See The Mathematica Book: Section 2.9.14.
See also: Raster, RGBColor, Dashing, Thickness, PointSize, StyleForm.