From what I gather from a couple of newspaper comments the system at fault is supposed prevent the plane from stalling and that a speed sensor might have been at fault. No more information was given.
As far as I know, stalling is when the speed of the plane is so slow that the wings provide insufficient lift and the aerodynamic control of the plane is lost. A typical result is falling in a horizontal (flat) spin which is very difficult to pull out of.
The solution is to detect the low speed early and point the nose down so that the plane gathers speed. The plane can then pull out of the dive when sufficient speed allows normal aerodynamic control.
This system seems to have operated during normal flight for no apparent reason, maybe due to a software bug, perhaps a bad reading from a speed sensor, etc. The system seems to have pointed the nose down (for no reason) and it never registered normal speed, so the plane kept trying to increase air speed with the result being a vertical dive straight into the ground.
Most media coverage seems to be avoiding the details, perhaps in an attempt to minimise PR damage to Boeing but they will have to come clean eventually.
