Thread tape is a flexible non-adhesive fiberglass thread that has been coated with Polytetrafluoroethylene (PTFE). Thread tape is often called Teflon tape, though the trademark owner of Teflon, DuPont does not manufacture a thread tape. The tread tape provides lubrication when connecting two pieces of pipes and fills in any minute gaps in the threads to prevent leaks.
When connecting gas lines you need a yellow thread tape that is approved for gas fittings. When you visit the plumbing section of your hardware store, ask for PTFE tape. The thread tapes are color coded in relation to their uses.
Thread tape is ideal for semi-permanent connections. When you replace an appliance, the gas connectors always need to be replaced. Using thread tape will allow you to get a good seal between connections but does not have any adhesive properties, which makes replacement simpler.
Thread tape should be applied to all male thread ends except when the end is flared. To apply the tape, hold the tape at the bottom of the threads with one hand. Wrap the tape in the direction of the threads of the connector until the threads are covered--usually two to five wraps of the tape, overlapping the tape slightly. When inserting the male end into the female end hand, tighten it with a wrench until it is tight. Do not over tighten or unscrew the connectors as this may damage the tape.