Devotee is a fancy word for "fan." Some people are superfans of Katy Perry. Others save their love and adoration for Mozart. Either way, they are huge devotees of the musicians they love.
People are called devotees because they're particularly devoted to something. Be it hot yoga, fluffy cats or chocolate cake, a devotee is an ardent follower. In the traditional sense this word was reserved for the extremely devout, who were devoted to their religion. But these days you can be a devotee of just about anything.