This is what it looks like...
Two needles held together by a small plastic shank. It fits in your machine just like any other needle, and they come in different widths shown by the number on the package.
You can thread with 2 different colors at the same time! If you want to use the same color in both needles, don't bother buying 2 spools, just use an extra bobbin on top. Thread the machine like normal with both threads, and then split them just before threading the needles. Loop one in each little curly thread holder thingy (no idea what it's really called) and then thread each needle separately.
Now you're ready to start playing around. Fiddle with the tension, stitch length, fancy stitches, and anything else you've got. The ones above are just a few of the things that you can do! The third from the top is just your basic straight stitch. I forgot to take a picture of the underside, but the bobbin thread pretty much just zig zags back and forth between the 2 threads. It would be perfect for hemming things like T-shirts! (I haven't tried yet because knit fabric and I aren't good friends, but I'm planning to make another top like this one, so I may give it a try soon.)
I was so excited, that I used this needle to hem my new keyhole dress instead of bothering with top-stitching, and it turned out great!