Washers are the main currency in Metal Arms: Glitch in the System. They are literally just like washers in the real
world, they can be used to purchase weapons, health, and ammo at Shady and Mr. Pockets' shop. Silver washers are worth one washer and can be found by destroying Mils and other destructible objects throughout the levels. There are "oily" washers worth anywhere between 10-50 washers which appear as a bright red shaded washer.
The amount of washers that can be found in a level is generally a fixed amount, but the amount dropped can vary. Specific mils will only drop washers, but the amount they drop will always change.
If you're the avid gamer you probably don't think much of the game's currency, but a lot of people were left head scratching at the thought of using washers, that you collect. By killing enemies. Looting their corpse. For parts. To sell.