Linen Thread is a critical end-game material. It’s made at a Spinning Wheel from Flax and you will need the thread to make end-game equipment, like padded armor. Linen thread is the limiting factor in most end-game items because it can be hard to accumulate flax.
Spinning wheels slowly refine flax into thread and you will want to get the process started long before you need the linen thread. It will take several minutes to craft a few threads.