Simply put, dry brushing is a technique used by many to highlight raised details. In the subjects you model, drybrushing would highlight raised instrument panel detail, as well as side panel detail in the cockpit areas of WWII aircraft, but you can highlight just about anything...machine gun barrels, walkways, etc.
I use an old #2 round brush that I cut about 1/4 inch from the handle, so it splays out a bit... Dip the brush into a small amount of paint, and wipe the brush nearly clean on a rag, etc, so that only a very small amount remains on the bristles. I usually use quick light strokes so as not to over-do the drybrushing. I'm sure that there are tons of others with better explanations that I have. Hope this helps.