A wash is very watery paint, or rather one with a smaller amount of pigment than is normal for the color, although some makers do actually use a hyped-up amount of water to the same effect. It is made to pool more in the recesses of the piece being painted, like on and around bolt heads and in the other indentations and creases and such, to add to the worn-down look. Don't worry about being new to the terms and tricks, I am VERY new, I only know this because I paint Warhammer 40,000 models too.
On a semi-related note, that helm is great! Can't wait to see the mask finished now! I'm going to have to borrow Killzone from a friend, since I've never played.