Airdrops
From time to time an airplane will fly overhead and drop a few supply crates for anyone around to salvage. This is called an Airdrop. The items they drop, where they drop them, and when they drop them is completely random.
Sometimes you can find a Supply Signal inside one of the airdropped crates. These allow you to call in your own Air Drops by throwing the signal on the ground. A small cloud of green smoke will deploy, and the airplane will come a few seconds later. Make sure you throw the signal out in the open so the crates don't get stuck on top of a mountain or hard to reach area.