OK, I got it! You know, I've been a musician all my life, but it's always been classical, or military, or church, or brass band music. I never strayed into the arena of popular music, rock, country, or jazz before. Now with the Ukulele, I am playing a lot of those tunes. And, as reflected by this thread, I'm encountering new (to me) vocabulary and concepts. It's a lot of fun!
One exception: I have played in a couple of very good studio jazz bands, one at conservatory where Xavier Cugate's road band musicians and wannabes filled the ranks, and another at Monterey Peninsula College where a lot of good players from the Bay area were always present. But those were simply rehearsal bands and nobody ever talked about recordings or commercial performances. They probably knew what "cover" means, but it never got to me.