What a npt valve is in a water tank pressure system?

npt, national pipe thread, is a standard for threads on a screw. I (not a plumber) don't believe "npt valve" makes sense , but probably refers to a valve which has npt threading.

Yes, a NPT Valve would be a valve with pipe threads as opposed to a sweat valve which has solder connections. There are several other type of piping connections too.