What is the meaning and function of thread guide?

Thread guides lead the thread in this order: Thread Spool to Tension Discs to Take-Up Lever to Needle. A machine is threaded with the take-up lever in its highest position and the presser bar lifted. Generally, the thread guides provide a path that keeps the thread from crossing over itself. From the spool, any thread guides mounted on the top cover are first, back to front. On the face plate (front of machine) any thread guides that sit to the right of the tension discs are threaded before inserting thread around & between the discs. If there be any thread guide that is BOTH to the left of the tension discs AND right of the take up lever, the thread would go there next. My fairly typical Japanese Zigzag Machine does not have a thread guide between the tension discs and the take-up lever, but if there were one, I would call it the "Tension to Take-Up Lever Thread Guide." From there the thread passes right-to left through the hole of the take up lever, and then down through all thread guides mounted to the left of the take-up lever. Lastly, the thread goes down through a needle clamp thread guide and through the eye of the needle. Not a short answer, but I hope it addresses your question.