Military Supply Drops are FND air drops you can steal by following their yellow smoke. They carry gunpowder and Supremo-Bond, and ten of them in solo play unlock It's Raining Treasure!
When a drop is coming in, it marks its landing spot with yellow smoke. High ground helps, so climb a lookout or fly over the area and scan for the plume. Recon points also give you intel on upcoming drops.
Reach the drop before anyone else and loot it. Drop zones can be out in the open, so approach with cover nearby in case FND soldiers show up.
Drops are one of the few sources of both gunpowder and Supremo-Bond, which you use for weapon upgrades, gadgets and Supremos. Do them in solo play if you are going for the achievement.