An onomatopoeic reference to the drum sound effect heard after a punchline has been told (called a percussive sting). "ba dum" is the sound of the snare drum and "tss" the cymbal.
It is often used in a sarcastic manner after a cliched or bad joke has been told.
John: "My essay on the cold war has to be finished tomorrow, I'm really Russian to finish... sarcastically ba dum tss"