A wash is a very diluted mix of a dark paint, usually black, paint. What you use it for is simulating shadow and gathered grime in nooks and crannies of your kit. I use a black oil paint (pending on the colour I'm working on top of..) diluted with heavy amounts of turpentine. This is almost like coloured water. It should flow by itself along crevices. Ofcourse you can use acryllics, enamels or even water paints, whatever you feel most comfortable using.
Light drybrushing, as opposed to heavy, is when you leave just a hint of light colour behind on the details. Something juuuust noticable and brings out the details..