Here ‘mother‘ means something that gives rise to something else. If we do not really need a thing, the lack of it does not trouble us, but when the need for it becomes so great that we cannot do without it, we are forced to find a way of getting it. A good example of this was the invention of radar (radio detection and ranging), a method of locating invisible objects, used first in World War II to detect the approach of enemy aircraft and submarines, and later put to many peace-time uses. Radar might never have been invented had it not been for the threat of war. Necessity was the mother of invention.
