The rich are the wealthy people in society. If you daydream of owning multi-billion dollar corporations and driving around in fancy cars, then you dream of being one of the rich.
Rich is a cognate (meaning a word that sounds the same and means the same) with various of Germanic, Frisian, and Dutch words for "king." Now it tends to be an adjective meaning "abundant." That might be abundant money, as in your rich cousin Henry. A rich sauce is buttery and creamy and delicious. Rich land is fertile and abundant in crops. A rich idea has many possible applications. Rich coffee is full, dark and intense.