Numnah is a word like gymkhana and jodhpurs that was brought back to Britain from India.
In India it was a dust-cover or a cloth to protect something.
Strictly speaking a numnah as riding tack is a light weight cloth that goes under an English Saddle and it's purpose is to protect your expensive English Leather Saddle from sweat and muck from the horse.
A well fitting English Saddle really shouldn't need anything but a thin cotton numnah underneath and merely to stop your saddle getting dirty from the sweat of the horse.
With an English saddle the optimum is not to have anything under it at all. Don't forget with an English saddle you should not (ever) use thick cloths or saddle pads at all. Pads and thick cloths under an English saddle that fits will make it so it doesn't fit.
This is more critical with an English saddle than a Western because it has a reduced load bearing area. (Though IMO, no matter whether you ride English or Western, the saddle should fit and without the need to compromise by sticking quilted pads underneath.)
Nowadays though folks are manufacturing great thick quilted saddle shaped pads and calling them "numnahs" but really they're saddle pads and not at all appropriate for use under an English saddle unless by someone whose kidding themselves that they're going to make their ill-fitting saddle more comfortable.