Without necessities, there would not be any inventions. We invented computers because somehow we felt the need to having this object in our lives (in order to search more informations, to go faster, etc).
So the human necessity is the mother of invention because it is the cause of human creation.
It is a proverb, or an old saying. Basically it is saying when you need something bad enough that it becomes necessary, you are forced, by human nature, to find ways to make it happen. <span />