" "f4" id's the lens (it's max aperture", and "stoped down to f16" means to set it to f16.
The expression "stop" comes from past cameras with apertures that were opened/closed to preset metal tangs, or stops."
So in simple terms it means to adjust your aperture. If your lens allows you to shoot at 1.8, it's not going to be as sharp there. Compared to lets just say 3.5 for fun?