Vertical lift as the term suggests is the motion of the aircraft vertically.

Hover or Hover Still - For the drone to hover all four motors are supplied power via the Flight Control System (FCS) to equal the gravitational weight of the vehicle. 

Ascend or Climb Ascend - For the aircraft to raise the RPMs supplied by the flight control system exceeds the gravitational weight of the aircraft. This thrust causes the aircraft to rise.

Descend or Vertical Descend – Just as Ascend if the power to the motors is reduced so the thrust is less than the gravitational weight of the aircraft it will descend. The FCS will normally take control of this to descend when landing as too fast descent can cause the device to stall and fall.