The term Dun appears in a couple of ways as regards flies.
A DUN is the term for a newly hatched upwing fly, i would imagine this term is used as they flies are often pale pastel shades with dusky coloured wings.
You are probably seeing the word with regard to colour, here it means a neutral brownish grey colour, quite flat and dull. You will see Blue dun, honey dun, dark dun, light dun etc they are all different shades of materials, all of which you will find very useful for dryflies, normally quite soft subtle colours.
As for tying hackles, hard to describe with words, easy to show have a look at
http://www.flyfisherman.com/ftb/hwhackle/ This is pretty good , it shows dry and wet style of hackles.
Sandy