Can someone explain this? I don't get it.

Certainly, as jokes are by far best when they are explained.

In this context the overall discussion is about pitch in the context of music.

Here the jokester takes advantage of pitch having more than one meaning in English. One of the alternate meanings is to throw.

Next the joke selects a banjo and an accordion, two instruments that are less popular and thus more likely to be understood by the general populace as being disparaged, which is a critical component for the audience to correctly infer the alternate meaning of pitch.

You put it all together and we have this hilarious joke:

A perfect throw is when a banjo is tossed into the garbage and it finds its perfect companion in an accordion that has similarly been discarded to the same trash receptacle.

(Edit: stupid auto-correct)

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.