What does it mean? It means you can put this in your document:
+-------+ +-------+ | A |------>| B | +-------+ +-------+
And get this (and as a PDF drawing, not as an image, so you can copy the text from it):
So it's an awesome way to add simple diagrams to your restructured text documents that look good in plain text, on the web, and on PDF.
Aafigure can create lots of interesting drawings, check their docs for examples here
It currently requires aafigure from bzr (0.3 won't work).