The 737 MAX was produced with several differences from the NG. Many of these differences were obvious such as the new LEAP engines or the larger flight display screens. Some were less obvious but well documented such as the FBW spoiler system. It also now appears that some differences were almost hidden, certainly from the flight crew. MCAS is one such difference.
MCAS was introduced to counteract the pitch up effect of the LEAP-1B engines at high AoA. The engines were both larger and relocated slightly up and forward from the previous NG CFM56-7 engines to accomodate their larger diameter. This new location and size of the nacelle causes it to produce lift at high AoA; as the nacelle is ahead of the CofG this causes a pitch-up effect which could in turn further increase the AoA and send the aircraft closer towards the stall. MCAS was therefore introduced to give an automatic nose down stabilizer input during steep turns with elevated load factors (high AoA) and during flaps up flight at airspeeds approaching stall.
On the face of it this seems like a sensible, beneficial system. However following the accident to Lion Air MAX-8 PK-LQP on 29 October 2018, shortly after take-off, in which it appears that the Captains AoA sensor was faulty, it is believed that the MCAS used this erroneous AoA data to command nose down stabiliser which was not counteracted sucessfully by the crew until the aircraft impacted the water.
Boeing have been working on a software modification to MCAS since the Lion Air accident. Unfortunately although originally due for release in January it has still not been released due to both engineering challenges and differences of opinion among some federal and company safety experts over how extensive the changes should be. Apparently there have been discussions about potentially adding enhanced pilot training and possibly mandatory cockpit alerts to the package. There also has been consideration of more-sweeping design changes that would prevent faulty signals from a single sensor from touching off the automated stall-prevention system