A wash is a very thin mixture of paint. It may be 75 to 90 percent solvent and the rest is color. Washes seep into crevices and provide depth and allow you to pick details out of the shadows they create. The solvent may be water or alcohol with acrylics or water paints, or be mineral spirits or tupenoid for enamels or oil paints. Generally you mix your wash and with a fine pointed brush, touch the area to be washed. Capillary action sucks the thin paint along and into the crevices.
I mentioned using washes of color in one of my recent replys to you regarding making water. Many novice modelers do water by painting the base blue and sticking a ship on top. Then they are upset because it looks like a ship sitting on top of a blue block. If instead you paint the base a dark blue gray (Paynes Gray), then lay on several layers of thin paint (i.e. washes) of green or blue, the colors will build with the resulting appearance of depth. Mount the ship in one of the early layers then build several layers on top of that. The ship is now in the water instead of on the water