Cl2 + H2O <-------> HCl + HOCl
Yes this is an equilibrium question. At equilibrium the forward and back reaction will stay the same unless you change the concentration of the reactants or products. The reaction will then "shift" to counteract that change, for example if you add more chlorine to the above reaction the left side of the equation is unbalanced and the reaction works to decrease the amount of chlorine. The way it does this is by reacting the newly added chloorine and converting it to products (in other words "shifting the reaction to the product side, or the right side).
So you can see that the same thing will happen if you change the concentration of HCl. By decreasing the pH you are in effect increasing the hydrogen ion (H+) concentration, or making the solution more acidic. So you in essence have more HCl (or product) on the right hand side of the equation than you need. In order to decrease this newly aquired excess of acid the reaction will work in reverse to turn it into reactants (hence the reaction is driven back to the reactant side, or the left hand side) this is why the teacher would have said "the equilibrium has been shifted to the left".
In summary, in an equilibrium reaction, if you add something to the left side the reaction will be shifted to the right side. Conversely, if you add something to the right side it will shift to the left side.