You subtract the final volume from the initial volume in order to deduce the total volume of NaOH from the burette added to the solution in the flask. This volume is then multiplied by the conc. of NaOH is used to calculate the number of moles

First you need to know that pressure only affects gas molecules. So according to the number of gas molecules on both sides of the reaction.. When pressure is increased the equilibrium will shift to the side with less number of gas molecules So for example if a reaction has 2 gas molecules on the reactant side and 4 gas molecules on the product side, when pressure is increased equilibrium will shift to the left hand side of the reaction, the side with less gas molecules