What does arbitrary refrain mean?

Answer 1

In music, a refrain is an interlude that is neither the chorus nor a verse, like a bridge. 'Arbitrary' means that it does not relate to anything previous. I assume you're referring to the Bud advert here, where the band slow the chorus down and use only percussion instruments after the beardy guy tells them to 'throw in an arbtrary refrain' ? Whilst not being entirely arbitrary, it is indeed a refrain.