NPT stands for National Pipe Thread. The way it works is this. Say you have a barb that is 1/2"x1/4"NPT the 1/2" designation is for the OD of the barb so it will take a 1/2" ID tube. Now the 1/4" NPT would be the inside diameter of the pipe, sender, or threaded end of the barb. So if you have a 1/4"-18 NPT the actual outside diameter of the pipe would be 35/64". Follow this
link for a chart on other NPT sizes as well. As for the the -18 after the NPT size would indicate threads per inch. Now remember if you tap a 1/4" NPT hole you'll need a 1/4" MPT barb. NPT is the umbrella term for national pipe thread, MPT is male pipe thread , and FPT is female pipe thread.