It's a second spool. Some sewing machines have one spool, some have two, some have four. There's a pretty fancy one called a serger that makes a double-wide stitch that you'll usually find on the outer stitching of your coats or nicer shirts.
You learn lots about this stuff with a professional seamstress for a mother-in-law. She made my wife's wedding dress - 16 feet long with pearls and lace galore, and a detachable train for dancing later.
How 'bout them Huskers!