"Rivalis" literally means "one who uses the same stream", referring to farmers who shared a common source of water for their crops. This shared resource naturally led to competition, and the term "rivalis" came to describe anyone who competed with another for something, especially something scarce.
Over time, "rivalis" evolved into the Old French word "rival" and then into the English "rival." "Rivalry" simply means the state of being a rival or the competition between rivals.
So, the next time you hear about a rivalry, remember the original meaning of "rivalis" - competition for shared resources. The concept of competition for scarce resources is a powerful one, and it is still relevant today in many aspects of our lives.