Washing is a technique to create deeper shadows on your model (figure in your case), by using very diluted paint, usually of a darker hue than the base coat, that flows into recesses. To properly cover the topic, one should write a series of articles. Luckily for us, a lot of people already did. Looking into the painting section of the forums you can find more info. A search on You Tube for "model washing technique" will return numerous good videos to get you started and see how it is done in practice.
Hope it helps..