What is le Chatelier's principle?

Le Chatelier's principle states that a reaction at equilibrium will shift it's position of equilibrium in order to oppose any changes to it's environment.

So if your reaction is exothermic, and you cool your reaction down, it will shift it's position of equilibrium more towards the products to warm it back up again, and vice versa for an endothermic reaction.

Likewise, if your reaction produces a gas, and therefore raises the pressure of the reaction vessel, if you decrease the pressure in your reaction vessel, the reaction will shift more towards the reactants to bring the pressure back up.