The plus button is a small round button on the Wii Remote, Classic Controller, Wii U GamePad, and Wii U Pro Controller. It is equivalent to the start button on the Nintendo 64 Controller and the Gamecube Controller.
Its primary use is for pausing during offline matches. In Online matches, it brings up a menu asking if you want to leave the match. It is commonly used for transitioning from the character selection screen to the stage selection screen once the players have chosen their characters.
It is also sometimes used for ending matches early, space jumping in the Subspace Emissary, starting slideshows in the Trophy Gallery, hiding the display in Trophy Hoard, showing help in the stage builder, and various other things.
Additionally, the plus button is used in most Masterpieces, the exact effect of it depends on the masterpiece.