Stop down basically refers to aperture. If you're shooting at f/2.8, stopping down means any smaller aperture, such as f/4 or f/5.6. You see it more often w/outdoor nature and macro photography, when they want a lot of depth of field. Most of the time, they don't even start until f/8 and they're often hugging the f/16 or f/22 (some guys stick to f/11 since that's when distortion shows up on the sensor).
I can't see the EXIF data on the shot since it was stripped, but my guess is that you had some pretty crazy shutter speed that you could have given up on and stopped down for more depth (i.e. instead of shooting at f/5.6 with a shutter speed of 1/500, you could have gone with f/11 with a shutter speed of 1/125).
Also forgot to add that lenses come into play also. The amount of aperture blades and so on changes the look of the bokeh.