This is a baseball term which refers to a specific type of pitch by a pitcher that is considered their best, and most unhittable pitch. It could also be used to refer to a single great pitch. The term, "money," in this phrase is used as a synonym for awesome, great, fantastic, or "the one." A pitcher who has a special pitch they use to strike people out, or a pitch they always get hitters to pop into the air, would call that pitch their, "money pitch."
Some examples of the money pitches of Major League players include Nolan Ryan's fastball or Pedro Martinez's curveball. This phrase is not used in official baseball parlance; it is primarily used by fans, and sometimes sports broadcasters. An example of its usage would be, "Joe's money pitch is his change-up," or "Did you see that! It was a money pitch!"