The floorclip is an effect of the floor texture (as defined by the footclip property in a TERRAIN lump) on actors, cutting off the lower pixels of the sprites. This feature is used to represent players and monsters wading ankle-deep in shallow waters.
Actors need the FLOORCLIP flag to have a floorclip. This flag can be set and unset directly in GZDoom, or changed with the now deprecated A_ChangeFlag or dedicated A_SetFloorClip and A_UnsetFloorClip functions.
Some classes have special functions that manipulate an actor's floorclip to achieve special effects; for example SerpentGib1 increase and decrease it to represent rising up to the surface then sinking back, ThrustFloor use it to thrust into or out of the floor, and WraithBuried use it to rise from the ground.