A maestro is an artistic master: someone who is skilled enough to be considered an artistic genius.
Taking one music class or art class can teach you a lot, but it won't make you a maestro. Maestro (which comes from Italian) is reserved for people with an enormous amount of skill and talent. This word can apply to any type of artist — and sometimes, to people with impressive skills in other areas — but it's most commonly applied to musicians. Master composers, pianists, cellists, guitarists, and conductors are often called maestros.