For the water, HCl is being introduced into the solution over the time period, populating the water solution with H+ (and Cl- , as the solution is now Hydrochloric acid). The pH number decreases at a decreasing rate meaning the solution is becoming more acidic but slowing as more HCl is introduced the less "room" for it in the water (just a way of thinking of it, don't include in answer). At t2 the solution seems to reach a point where it is fully saturated with HCl, explaining the flatline of pH at 1.
For solutions X and Y, I believe the concept the question is testing is the buffer capacity of a solution containing a weak acid/base and its conjugate pair. This is because X and Y both contain the same ions and behaved similarly (resisting significant change in pH) until a point where the pH of X begins to drastically drop compared to pH Y. Firstly this tells us the nature of the solute in X and Y: a weak acid/base and its conjugate pair. The graphed behaviour also indicates that solution Y has a greater buffer capacity in terms of resistance to the introduction of acid (HCl) than solution X, as X can be observed to exceed its buffer capacity before Y does. This concept can be explained in further depth involving Le Chatalier's Principle etc., and I reckon have a look at it in the textbook if you haven't already done so. Overall I think the answer the question is looking for is for you to explain the behaviour of X and Y in context to buffer capacity, and conclude that the molar concentration of the ions (particularly the base ions) in solution Y is greater than those in solution X, giving solution Y a greater buffer capacity when an acid is introduced, explaining the graphed pH behaviour.
Of course youll have to explain these concepts in terms of the time intervals, and the balanced equation it is asking for could probably be either H2O +HCl ---> etc., or possibly the equilibrium equation of a substance which could be the weak acid/base in solutions X and Y.
Anyways take this as a rough guide but I hope I pointed ya in the right direction.