Scrap is the currency used while on the battlefield to buy and upgrade turrets. They are visually recognizable as they resemble a TV or monitor colored bright blue. To collect scrap just walk over it and it will be picked up.
The trench has the ability to pull scrap toward itself by pressing . There are also emplacements called Collection Prongs which will pull scrap toward itself and collect them. Only scrap that you and your emplacements collect will count toward this achievement
If you play through the game normally you should unlock this toward the end of the 'Pacific' act.