Tapered pipe threads rely on metal-on-metal contact to seal, which can still allow small fluid leaks. Teflon tape, or PTFE tape, solves this by filling the tiny voids between the threads as they are tightened. It acts as both a thread filler and a dry lubricant, allowing you to screw the fitting in further and tighter without binding or galling the metal.
When applying the tape, always wrap it in the same direction the fitting turns to tighten (clockwise). If you wrap it counterclockwise, the tape will bunch up and unravel as you screw the fitting in, leading to a leak. Keep the tape back from the first thread of the fitting. This prevents loose shreds of plastic tape from tearing off and entering the fuel or hydraulic lines, where they can block small orifices.