He explained that if pressure is increased by decreasing volume (if a piston pushed down on the reaction, for example) then the concentration of each of the gases in the reaction would increase. We calculate concentration by using
where n is moles of gas and V is the volume of the container. If the pressure doubled by decreasing the volume of the container by half, then the new concentrations would be doubled. If one side of the reaction has more moles of gas, then once we plug in the new concentration we would see a change in Q which would push the reaction toward the side with fewer moles of gas.
This does not happen when you increase the pressure by adding an inert gas. If you add an inert gas, the each gas' moles and volume do not change. This means the concentrations do not change, so nothing happens to the reaction.