Original: The Boeing 727 design was a compromise between United Airlines, American Airlines, and Eastern Air Lines requirements for a jet airliner to serve smaller cities with shorter runways and fewer passengers.
Simplified: The design of the 727 was thought up as a compromise between United Airlines, American Airlines, and Eastern Air Lines and their requirements for a jet plane to fly to smaller cities, many with shorter runways and less passenger demand than the 707, a bigger plane that was the only jet airplane made by Boeing at the time.