Flax Thread is spun from Flax at the Spinning Wheel: 4 Flax make 4 thread, with the Flax Thread technology and Farming mastery 3. It is the string in every fishing rod and bow, the wick in every candle, and what the Weaving Loom turns into Fabric (4 thread for 1).
The Tailor sells thread only after the upgrade that needs it. The town Tailor stocks 40 at tier II and 80 at tier III, but going to tier II costs 40 thread. Spin the first 40 yourself, or buy from Herm, who carries 20 from his third tier with 15 copper added to its base price of 15.
A zombie at the wheel can spin more. The zombie Tailor perk adds 2 thread a batch.
Studying it costs 3 Science and gives 15 red and 30 green.