Zach,
Drybrushing is a technique used to lightly go over an item with the intent on adding a slight color variation to it.
I could demonstrate it easier than describing it, but here goes; when I paint a building, the last step I do is I take a wide brush, dip it in a light tan color, then go back and forth on a piece of paper until about95% of the paint is off the brush, hence the term drybrushing.
I then lightly go over the building with the almost dry brush and this allows me to aply paint to the building to highlight it, add a weathering to it, etc, etc.
It is commonly used to go over a tank once it's finished to add weathering to it.
Hope this helps........