PTFE seals are small rings or packing material made from PTFE (the same material as non-stick cookware) that seal the needle as it passes through the airbrush body. Unlike rubber O-rings, PTFE is chemically resistant to solvents, making it the preferred seal material in airbrushes used with lacquers, enamels, and other harsh solvents that would degrade rubber.
When a PTFE seal wears out, you’ll notice paint leaking back into the handle section of the airbrush, or the trigger feel changing (becoming stiffer or looser). Replacement PTFE seals are available from airbrush manufacturers and are part of a proper maintenance kit.
Related: O-Ring · Needle · Deep Clean