It's an onomatopeia for a drum technique normally accompanying the conclusion of a cheesy joke or a comedy pratfall (where someone is made to look like an idiot by their own devising - such as falling on a banana skin they earlier discarded). It consists of two fast rimshots and a splash cymbal - producing the sound "badum tish".
#1: What does bees do if they don't want to drive?
#1: Wait at the buzz stop! Haha!
#2: Badum tish!
that sound after someone makes a corny joke
two drum sounds and a cymbal
Why'd the cookie go the hospital?
Because he was feeling crumby!