This is a 356 metre reel of artificial flat sinew hand sewing thread. This thread is a modern replacement for sinew or 'catgut' thread.
Artificial sinew is often used to stitch up historical style leather craft projects and those with an American Western style. It teams up very well with veg tan tooling leather.
We'd suggest that you use No 2 Needles to stitch this this thread. It's heavily waxed and pale yellow in colour.