The 1v1 multiplier is a mechanic in Super Smash Bros. Ultimate designed to speed up matches between two players. In short, when a game has only two players, and items are off, characters take 1.2× damage from all sources.
The 1v1 multiplier is a damage-taken modifier rather than a damage-given one, and is applied after all other calculations. For example, a move with a damage output of 10% will inflict said amount of damage, and then once all other effects are determined (such as the amount of knockback), an extra 2% will be added. As a result, it has a few unintuitive interactions: