How does Le Chatelier's principle work when the pressure is changed?

First of all, it is important to note that changing the pressure will only affect the gas molecules in the reaction. Taking a simple equilibrium equation such as:

2SO_{2 (g)} + O_{2 (g)} <===> 2SO_{3 (g)}

From the state symbols in brackets, we can see that all of the molecules are gaseous. There is a total of 3 gas molecules on the left-hand side of the reaction (2 SO₂ and 1 O₂) and a total of 2 gas molecules on the right-hand side of the reaction. Le Chatelier's principle says that for any change made to the reaction, the equilibrium will move to counteract the change. In this case, if the pressure is increased, the equilibrium will move to reduce the pressure. This is done by shifting the equilibrium to the side of the reaction with the least gas molecules (the right-hand side in this example). This reduces the overall number of gas molecules in the reaction and therefore reduces the pressure.