If I've got it right, Mach crit is that mach number above which supersonic flow will appear over a part of the wing, where depends on the wing profile. When this happens, you will get Mach buffet caused by flow separation at that point.
One of the aerodynamic effects experienced at high Mach numbers is the aftwards movement of the Centre of Pressure, causing the nose to dip, Airspeed to increase and therefore, Mcrit to be exceeded leading to buffet, increased drag and, possibly, unusual control effects. The Mach trimmers counter this dip to avoid this exceedance.
Hope that explains the basics.