What happens to the equilibrium when the pressure increased

When a reversible reaction is occurring and the pressure is increased then the equilibrium which is dynamic will shift in order to facilitate this change by virtue of Le Chatelier's principle.  When pressure is increased, the equilibrium shifts to favor the side with the most number of moles.

For example, the theoretical formation of ammonia is characterized by the reaction:   N₂   +   3H₂    ⇆   2NH₃ 
If the pressure is increased in this reaction system, then the equilibrium would shift to the left, since that side has three moles of hydrogen and one mole of nitrogen for a total of four moles while the right side only has two moles.