Someone masterful is really good at a particular task or skill. A masterful pianist is accomplished and experienced — in other words, she plays extremely well.
A masterful stone mason builds beautiful, sturdy walls, and a masterful math teacher is capable of teaching even students who think they hate math how to solve complex equations. An alternate way to use this word is to mean "like a master" or "controlling and powerful." In fact, this is the word's original meaning, from the Latin root word magister, "chief, head, or director."