To simplify it, PostScript is same as PDF. :) It is just a list of commands to the printer (or PostScript (PDF) viewer), telling him what to do. Here is example of postscript commands (from wikipedia), which prints 'Hello World':
%!PS
/Courier % name the desired font
20 selectfont % choose the size in points and establish
% the font as the current one
72 500 moveto % position the current point at
% coordinates 72, 500 (the origin is at the
% lower-left corner of the page)
(Hello world!) show % stroke the text in parentheses
showpage % print all on the page
As you can see this is something similar to a "turtle" draw commands, but taken to a professional level.
Normal desktop printers are able to interpret this set of commands to print whatever you want. Wherea Zebra printers do NOT need to print whatever you want. They just need to print something simple like receipts, barcodes, labels. Thus there was no point to put more expensive chips into them which would support PS. However all of Zebra printers can print IMAGE. (Meaning graphical bitmap, usually a monochrome image).
So the solution to your question sounds like this:
Despite it is NOT possible to send directly PostScript commands to Zebra printer, you can always render your post-script command into image/bitmap (like Adobe PDF reader does) and then convert this image to monochrome and print it using any Zebra or other thermal printer.