Feet Per Second, referring directly to the feet per second the projectile, or bullet, travels after leaving the barrel. Otherwise known as velocity.
For example, the average 9mm 115 grain commercial load, leaves the barrel at 1150-1250 F.P.S.. Then as is travels away from the gun, it continues to lose velocity.
So if this bullet left at 1150 fps it would likely be travelling at 1050fps at 50 yards, then down to 950 fps at 100 yards.
Lighter bullets tend to lose less, and heavier…..