@monkey999 - Hit boxes and hurt boxes are rectangular regions which overlap a player's character in a fighting game. For example, if your character throws a punch, then when the punch is active (i.e. briefly) then there will be a hit box. If this hit box intersects with a hurt box of the opponent, then the attack is judged to have hit the opponent and will cause them damage.
If you're standing too far away from the opponent and do an attack, although the hit box for the move will exist, it will not intersect with the opponent's hurt box so it won't count as a successful attack and won't do any damage to them.
Further reading if you're interested: Understanding the mechanic of frames, hitboxes and hurtboxes