The minus button (or - button) refers to the small, white button on the left side of the Wii Remote, Classic Controller, the button on the right side of the Wii U GamePad and Wii U Pro Controller, or the dark gray button on the left side of the Nintendo Switch Joy-Con and Pro Controller. On the Classic Controller, Wii U GamePad, and Wii U Pro Controller, it is referred to as the Select button.
When playing Super Smash Bros. Brawl with the Wii Remote and Nunchuk, the - Button has absolutely no function. However, when playing with the Wii Remote held sideways, the minus button can be used to grab.
In Super Smash Bros. 4, the minus button returns on the Wii U GamePad, still doing nothing. It can still be used to grab with the Wii Remote held sideways.