[ryukumon]

A wash is basically paint that has been thinned down a lot, enough so that it runs into lines and pools into the valleys of whatever you are painting. Plus afterward you can wipe off the excess and just leave behind the paint in those lines and valleys. There are a few tutorials on youtube about them.

Check out 'panel line' or 'panel lining' tutorials for different methods since panel lining is a form of wash.