Wikipedia tells us that a paint wash is a painting technique in which a paint brush that is very wet with solvent and holds a small load of paint or ink is applied to a wet or dry support.
Essentially, you thin the paint before applying it. You can even apply more than one color to get a cooler look and feel. As usual, I didn't know what this was until Audrey advised Shane that she was going to put a wash on some things he built. I immediately looked it up, but then I started looking forward to seeing what Audrey's wash would look like.
So now here's what a paint wash looks like to me:
It's nice, isn't it?!