There are five difficulty levels: Easy, Normal, Hard, Expert and Master.
For some songs there is also the option to play on Append difficulty, which requires at least three fingers to play.
*Make sure to disable any functions on your device that use three or more fingers (e.g., gestures on iOS).
The Master difficulty needs to be unlocked for each song by meeting certain requirements.
Tap the Master icon on the song selection screen to check the requirements for unlocking that difficulty.