https://www.cnet.com/news/boeing-details-fixes-to-get-737-max-flying-again/

Though the official causes of both accidents have not been determined, investigators have found clear similarities. Under scrutiny is a flight control system called MCAS (for Maneuvering Characteristics Augmentation System) that pushes the 737 Max's nose down when it determines that the aircraft is about to stall. Preliminary data from both crashes show that flight crews struggled to take control as the airplanes continually dove just after takeoff. 

Boeing said the fixes will "reduce the crew's workload in non-normal flight situations and prevent erroneous data from causing MCAS activation." Among them are: