The traditional water-soluble embroidery stabiliser. Clear dense film. Cold water soluble (lukewarm is ideal).
Simply embroiderer your design onto the film then wash away. Designs can be drawn onto the film for guidance during stitching - naturally, the clear film is ideal for tracing designs.
Very popular product but being widely superseded by Vilene's Solufleece. Solufleece is much easier to work with.