Manifold Pressure is a measure of the suction                                                               

Every time the piston drops into the “intake” stroke it is literally pulling or sucking air and fuel into the cylinder                                                                                            

At idle power the MP gauge might read 15 to 16 inches (outside may be 30in)              

The numbers are in inches of HG; low number is not sucking as much into the engine a high number sucking means more fuel and air are entering the engine.       

Less suction means less fuel through the venturi (still the near perfect mix only less power)                                                                                                                      

As you need more power you lift the collective. This opens the throttle removing the restriction, and allowing more fuel and air to be sucked into the cylinders for more power