The human received a message on the phone, didn’t like it, and “barked” at the phone. The dog considers this funny, considering what the human had just said about “barking at nothing”.
This, I understand, is the original: Und der Haifisch, der hat Zähne / Und die trägt er im Gesicht / Und Macheath, der hat ein Messer / Doch das Messer sieht man nicht.The English translation is pretty well-done. But I’ll try a more literal translation, anyway: And the shark, it has teeth / and it wears them in its face / And Macheath, he has a knife / But the knife can’t be seen.
See also: Streisand effect.