Traditional Boeing 737, notice the location of the engine under the wing
Boeing 737 Max with the engine more forward ahead of the wing
With the engine moved forward they created a more nose heavy situation. The nose is controlled by the elevator in the tail rudder, moving the elevator up and down will move the tail up and down but also will raise or lower the nose of the aircraft.
Boeing was concerned about pilots getting into a nose position that would cause a stall. The nose is raised too high and the wing cannot support lift and you stall, fall out of the sky. They built into the software a program called MCAS, which will take control away from the pilot without his knowledge. In fact the MCAS was not mentioned in the pilot's 737 Max handbook except for the definition and they were never told or trained for MCAS. The MCAS is counteracting the pilot's commands and they get into a porpoising situation and the pilot loses control.