Pitch is another word for throw. The joke is that a perfect throw is one where a banjo Is thrown into the trash (this is already funny if you consider how careful musicians are with their instruments), and what makes it a perfect throw is that it lands on an accordion (the fact that the accordion is already in the trash is also funny) and the fact that one shitty instrument landing exactly on another shitty instrument would be an athletic achievement that is given the name of a desirable musical ability “perfect pitch”, is excellent.