PTFE (polytetrafluoroethylene) pipe thread tape. Used for sealing tapered thread joints, such as NPT.
Starting on one end of the threads, wrap the male threaded piece with this tape in the same direction as the fitting will be turned to connect to the female fitting. Keep it taught while applying and overlap the tape each time around by a 1/4 to 1/2 the tape width. The tape fills in small gaps between the threads and compresses when tightened making the joint leak free. Wrapping the wrong way usually results in the tape unwinding and possible leaks.
PTFE thread seal tape by BE Pressure Supply