The Max is the latest upgrade to the Boeing 737, which has been flying since the late 1960s. Because its engines were larger and heavier than on previous 737s, they were placed higher and farther forward on the wings. That created concern that the plane might be slightly more prone to an aerodynamic stall if not flown properly, so Boeing developed software to prevent that.
MCAS was required to prevent the pilot from pulling up too much, and it pushes the nose down.