Kreinik Japan Threads are synthetic, non-tarnishing gimps (wrapped threads) that give the appearance of stitching with real metals. Since real metal threads are rare and expensive, Japan Threads are an ideal substitute. They offer a smooth, high sheen. Japan #1 is the thinnest, and is most commonly used to couch thicker Japan #5 and Japan #7. Kreinik also makes Japan Gold and Japan Silver in Braid and Ribbon sizes, which can be couched with Japan #1. 

Couching is basically just tacking down a thread, usually from side to side. Lay the thicker thread on top of your fabric or design (plunge the ends to the back or fold under on top), then use the thin thread to tack it down in your desired pattern. With some threads, like Kreinik Japan braids and ribbons, you can place your couching stitches right through the braid or ribbon. Japan #5 and #7, however, are solid threads and must be couched from side to side.