Definition
Stropping[1] is the process of polishing the edges of an already sharpened blade using a strip of canvas, leather, or other fabric such as felt. Unusual sources for stropping surfaces can be used (newspaper being a common example). While stropping can be used to finely sharpen and polish any blade, it is nearly always used to maintain straight razor edges, used immediately prior to (and usually immediately following, for the purposes of cleaning the edge) the shave.
Video
For a video demo of how to strop single-edge and double-edge blades, plus cartridges, GemCutlery has a good video [2].