Speed, abbreviated as (SPD) is an in-game Variable that indicates how many units Mario is moving. Mario's vertical speed is called Vertical Speed (VS) and his horizontal 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" instead of the minus symbol -).
Gravity effect and Terminal Speed[]
Mario falling at Terminal Speed (M75) in PSS
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 also unused when Mario is on the ground, so his VS is set to 0 as soon as he touches the ground, and remains that way until Mario is airborne again.
However, there are movements that don't reset Mario's VS to 0 when he touches the ground (See Vertical Speed Conservation).