What is MCAS?

And why is Boeing’s Maneuvring Characteristics Augmentation System a problem?

Boeing had a problem with the 737 MAX, and it was not a new problem. Rather, it was one of several awkward characteristics that dogged the manufacturer since the 1960s, when the 737 originally entered service, and which has confronted engineers working on revised generations of the aircraft for some fifty years: the aircraft is too low to the ground for modern engines.

The 737 was designed to have steps fold down from the aircraft for easy boarding and disembarkation — no costly jet bridges required here — and for easy ground loading and servicing of the aircraft.