A ground hitbox exists when a character is properly on the surface of the ground. This does not include the state of being knocked down.
In typical 2D fighting games, so long as one never inputs upward on the stick (indicating a jump), one will almost always have the ground state. Basically, in this state, the damage you receive will not have any damage correction taking away from it, so the bulk of damage you take will be in this state.
Notably different in KOF, though, during small and medium jumps (with an air hitbox), when you take an attack, it gets treated as a counter hit.