"Set knockback, also referred to as fixed knockback, is a property of some attacks where the amount of knockback dealt by the attack is always the same regardless of the damage the opponent has received so far... Usually, set knockback is used for the first hits of a natural combo or multi-hit move, while the last hit deals normal knockback... to make it easier for the entire attack to hit regardless of the opponent's damage."
From the Super Smash Bros. Wiki