An origin myth is a story conceived by humans to explain the causative factors behind existence. The word "myth" itself derives from the Greek word "mythos," meaning "story-of-the-people."
The ancient Romans also had an origin myth to explain the founding of their Republic more specifically. In that story, Romulus and Remus, a pair of twin infants, were abandoned by their parents in the River Tiber. Discovered there by a she-wolf, the babies were raised by her for a short time until a shepherd discovered them and raised them as his own.
Upon reaching adulthood the brothers sought to honor their wolf-savior by building a city by the Tiber, but could not agree on a location. As a result, Romulus slew Remus and founded the city himself in 753 B.C., naming it Rome after himself.