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. On the N64 controller, it is used for taunts. On the GameCube, Classic, Wii U Pro controller and Wii U GamePad, it is used for shielding, rolling, air dodging in midair, and sidestep dodging (which is shared with the R button) while on the 3DS, Switch Pro and dual Joycons, it is used to grab.
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.