A "Wash" is a highly thinned enamel or oil paint that you apply after the model is almost complete, to bring out hidden lines and surfaces. It is mostly done with Burnt Umber, Black or a darker shade of the model's base color.
After you have completed your model, you spray a gloss coat (people even use Future floor wax) over the entire piece. It aids in letting the wash "flow" easier over the kit and also protects the paint underneath. After letting the wash dry several hours, it is removed with thinner or mineral spirits, leaving the wash in the panel lines, around bolts and such. You can use a cotton swab, dipped in the thinner, to remove unwanted wash from the surface. You then give the piece a dull coat, to remove the shine.
Remember, when doing a wash, you do not paint it on, you let it flow off the brush and sink down into the recessed lines.
An example of how I do a "Wash" is found here: http://www.erock68.com/erock68t15.html