I've never seen that prefix, but it is representing the mounting tabs (or pins, or clips, etc) for the header. The tabs are soldered to pads (or plated-thru-holes) on the PCB.
Sometimes the mounting features are only there for mechanical strength. In that case, they are soldered to the pcb but they don't need to connect to anything electrical.
Often, however, the tabs are connected to a metal chassis and also serve as EMI shielding. Then the pads (or holes) on the pcb should be connected to ground.
Look at these two pins on a USB connector. They are thicker than the others, and are also part of the metal chassis.
In your schematic, it appears that MH1/3 are connected and so are MH2/4, but this is just the way they were drawn. Each of these is connected to ground (the red triangle), so in reality they are all connected.
"Mounting Hole", perhaps? "Mounting Hardware"? "Header Mount"?