It's actually called Thread Heaven and I'm probably the last person on earth to find out about this stuff. It's a synthetic thread conditioner that stops tangling and fraying. Seriously. Run the thread on your needle through this and it makes a slick coating that works like ball bearings. Works especially well on metallics, silks, and rayon threads to make them more pliable and easier to use. Thread Heaven is chemically inert and will not react with the fibers.
So yeah, thread lube, right? Makes stitching more pleasurable. Especially with those pesky metallics.