Speed
Speed, abreviated as (SPD) is an ingame variable that indicates how many units Mario's moving. Vertical Mario speed is called Vertical Speed (VS) and Horizontal Mario speed, Horizontal Speed (HS). If the Speed value is Negative, it means that Mario is moving Downwards in VS, or Backwards (prefixed in both cases with an "M").
Gravity Effect and Terminal Speed
When airborne, mario's Vertical Speed is constantly decreasing by 4 every frame, as Gravity is applying on him a Constant of -4 per frame. Additionally, Mario's Vertical Terminal Speed is -75 (M75 as shown in-game), so he will not fall faster than that. VS is unused when Mario is in a ground state, so his VS is normally reset to 0 as soon as he touches the ground, and will remain that way until Mario is Airborne again. However, there are certain movements that don't reset Mario's VS when he touches the ground (See Vertical Speed Conservation).