An MRB is a formally chartered group of people empowered to supersede or direct the change of certain aspects of published Engineering during production. The MRB concept is important to SE because under certain circumstances, the board members can authorize temporary or limited invalidation of qualification data. The process is not normally tied to the SE process, so SE practioners need to be wary about the long term implications of having qualified a design vs. having accepted a S/N of that design by way of an MRB action.
The MRB process is actually far more thorough than the simplistic notion described above. I’ve addressed only enough to lead to my cautionary point.