The minus button (or - button) refers to the small 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 Wii Remote, Classic Controller, Wii U GamePad, and Wii U Pro Controller, it is referred to as the Select button and matches the colour of the controller.
When using a horizontal Wii Remote, the minus button is used to grab. When using the left Joy-Con horizontally, the minus button acts as the Start/Pause button. The minus button does nothing on other controllers.