The Poké Ball Plus is an optional controller, shaped like an actual Poké Ball from the Pokémon games and cartoon, which you can play this game with. You can purchase it separately or as a bundle with the game. At the time of writing, it can’t be used as a controller for other games (but is compatible with Sword and Shield).
The controller itself features an analogue stick in the middle, where the red and white halves join. You can click the analogue stick inwards to act as the main button. There’s also a second button at the top of the red half, which acts as the secondary button.
Additionally, the controller has built in gyro sensors, allowing for motion controls. This is primarily used when catching Pokémon, where you can mimic the motion of throwing a Poké Ball, causing you to throw a Poké Ball in the game. You can also shake the controller, which acts as a third button.