The L button refers to the left shoulder button on the Wii U GamePad, the Nintendo 64, GameCube, Classic, and Wii U Pro Controllers, and on the Nintendo 3DS itself. Similarly to the R button, it is used in Super Smash Bros. Melee and Brawl for defense techniques, such as shielding, rolling, and sidestep dodging, while it is used in the original Super Smash Bros. to taunt. In Super Smash Bros. for Nintendo 3DS, due to the 3DS lacking a Z button and its R button being used to shield, the L button is instead used to grab. In Ultimate, it also refers to one of the shoulder buttons of the Pro Controller or the double Joy-Cons (not to be confused with the SL button). By default, it has the same function as the R button, being used for grabbing and, while in the air, airdodging.
When using a single Joy-Con in Ultimate, the L or R button (depending on which Joy-Con is used) is not set to any action by default. Instead, SL is used for grabbing and SR for shielding. In this configuration, the L, R, ZL, and ZR buttons cannot be set to grab.