Teflon® tape (polytetrafluoroethylene, or PTFE), AKA plumber’s tape, is a thin film used to seal pipe threads. The tape is durable, flexible, and fills in the minor imperfections in the threads to seal leaks.
To use, hold one end of the tape against at the start of the pipe threads/end of pipe and start wrapping opposite of the direction of the threads, keeping the tape flat. Pull the tape tightly into the threads but not so tightly that the threads cut through. Keep going in a flat uniform manner until you come to the ends of the threads/pipe. Now thread the pipe in carefully and you’ll have a good seal.
You want more than one layer but generally not more than three or four. If you are using drastically different materials, such as copper or brass to PVC, you may need extra but that is unlikely. Too much tape is as bad as too little. It will prevent a good seal. With the proper amount the pipe should thread together smoothly, sealing all gaps in the thread.
When do you not use plumber’s tape?
This tape is only effective on tapered threads as it uses compression to form the seal. Fortunately, the vast majority of the connections you make in irrigation will be with National Pipe Thread Tapered Thread, or NPT, and this tape will work well.
Many will tell you that brass to brass fittings are self sealing. In one manner they are but it takes an experienced professional to know which ones will work. You risk a leak if you are not sure you have the correct condition. Tape is cheap insurance. Brass fitting threads also tend to be extra sharp, especially on the smaller pieces, and may cut through the tape and ruin the seal. In this extra care is needed. In extreme cases you might need pipe dope but that is rare. The general connection involving brass in an irrigation system is with the backflow. Plumbers tape should work just fine there.