Hi Peter, 'stippling' refers to the application of any type of paint by using a 'stabbing' motion vertical to the subject as opposed to brushing. You use a round brush that has had the point cut off to leave a flat end. Press the end into your paint then 'stab' the flat end onto a piece of paper etc., to remove the excess, then stab the brush onto the area you wish to stipple. I use this technique a lot for horses, weathering and 'five o'clock shadow'. You can find lots of videos on YouTube. Hope this helps.
Regards
Ron