An attack's knockback is how far it can send an enemy. For example, Bowser's forward smash is an attack of high knockback; it launches opponents very far, so far that at high damages it's more powerful than a home-run bat smash attack. Pikachu's neutral a, on the other hand, has very low knockback - it hardly sends opponents anywhere.
The knockback of most attacks increases as the target's damage increases; however, there are some exceptions, notably Fox's shine. Attacks like this are said to have set knockback. Also, a heavy character will be less knocked back from an attack than a light character.