This thread is called artificial sinew and is a rot-proof replacement for natural animal sinew.
Artificial sinew 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, but it can be easily split down for lighter use.
We'd suggest that you use No 2 size Needles to stitch the full thickness, or No 4's if you split the thread. It's heavily waxed and a yellowy-beige colour.