Hi, Merry - I always thought it was the "dirty parts" of a song, garbled up so people couldn't understand them. Not so. It's a musical tool called a burden, a nonsense line or verse that's used for its musical value and to fill in the meter. Somebody gave a great explanation here, and I wish I could find it. My dictionary says burden, chorus, and refrain are just different words for the same thing, but they aren't. If I recall correctly, a "refrain" is a single line, repeated at the end of every stanza of a song. A "chorus" is a number of lines, repeated at the end of each stanza. And if I have this correct, a "burden" is a chorus or refrain made up of nonsensical words or sounds.
OK, you music pros, did I get it right?
Merry, I'd suggest you check this thread tomorrow or the next day, after the pros have awakened and had time to give a solid answer.
-Joe Offer-