Droop on an RC car is the amount of down-travel a suspension arm has — how far the wheel can drop below ride height before the suspension tops out. It's limited by a downstop screw.
Droop is set with the downstop screw or the shock’s stroke and measured in millimeters, but there are several ways to measure it — chassis to wheel nut, chassis to hub, or shock length — so a droop number is only meaningful if you and whoever gave it to you measure the same way. CompetitionX makes exactly that point. At the front, less droop gives sharper steering, less traction rolling and more high-speed stability on smooth tracks; at the rear, more droop is more stable over bumps. Front and rear are set separately and should match side to side. Full-size cars use the word for the same thing, down-travel, but RC racers adjust it with a screw between rounds.
Droop changes weight transfer at both ends, so a couple of millimeters can flip a car from stable to darty — and a droop number measured differently than your notes is worse than no number at all.