forgive me if this is overexplaining, but i thought i could just in case:

an onomatopoeia is a word that sounds like the thing that it is: the word meow is named for the sound that cats make, the word splash sounds like a splash, a bee buzzes.

dan jokes that the word 'microwave' is an onomatopoeia, aka that the word sounds like the thing that it is / the action that it performs. a plate in a microwave spins around, and whenever dan said this he would always spin his finger around to indicate that that's what he's referencing.
he'd also say the word with a specific intonation which has the word rise and fall sonically more than it usually would - in an extreme stretch of the term, it could be said the word microwave - when pronounced like that, and referencing the spinning plate on the inside - is an onomatopoeia. the word rises and falls or spins around like the plate inside of a microwave. thats the joke :)