Has this ever happened to you? You start descending your brand new quadcopter from altitude, with its quite advanced (and expensive) camera and gimbal system. You try to slow the descent by applying some throttle. But instead of slowing, it starts to wobble in air. You apply more throttle to stop the descent, but that just makes it wobble and drop even faster. You're finally at full throttle as it smashes into the ground (or submerges in some lake, river, or ocean). You curse the maker of the quadcopter for selling you such an obviously defective product. But the product was not defective. You've just encountered a common aerodynamic phenomena that is particular to all rotorcraft.
As mentioned, Vortex Ring State (VRS), aka Power Settling, aka Blade Stall, is common to all rotorcraft. It's encountered when a rotorcraft descends vertically too quickly. From this point on, let's assume that the rotorcraft is a quadcopter. The quadcopter's propeller blades may descend into the turbulent downwash beneath the craft. If this occurs, the blades lose some lift, causing an even faster descent into the downwash. A vortex also starts to form in a circular ring (the Vortex Ring) around the blade's path of rotation. This vortex suck turblent air from beneath the blades to the top of the blades. Applying throttle just increases the vortex ring, eventually causing total loss of lift from the blades.
This is obviously an extremely dangerous situation to encounter for manned aircraft, such as helicopters and other multirotor aircraft. But there are ways to avoid it, and if it can't be avoided, there are ways to attempt to recover from it. The following video discusses VRS in more detail, along with avoidance and recovery procedures for your quadcopter.