inch NPT refers to the standard screw thread size most airbrush compressor manufacturers have settled on for air hose fittings. It’s a defacto standard.
If your compressor doesn’t use this size fitting, available in any store selling standard US plumbing supplies, contact the manufacturer about the thread size and how to order parts and adapaters.
This particular piece of trivia is useful if you ever need to adapt airhoses, connect them in series, or find replacement parts for your compressor.
If you’d like to know exactly what inch NPT means try perusing Wikipedia’s article on the subject.