This thread is called artificial sinew and is a modern replacement for sinew or 'catgut' thread, which despite its name, used to be made from the intestines of farm animals, most usually sheep or goat.
Artificial catgut is often used to stitch up historical leathercraft projects and those with an American Western style. It teams up very well with veg tan tooling leather.
This is a 28 yard reel, which stands just over 2" (5 cm) high. The thread is flat and quite heavy weight. We'd suggest that you use No2 Needles to stitch this this thread. It's heavily waxed and a yellowy-beige colour.