In past elections, both Liberals and Conservatives in Canada have won over 50% of the seats with less than 40% of the seats, and this difference is caused by our first-past-the-post (FPTP) electoral system, which is one of the first voting systems ever invented, and is still the law of the land today.
FPTP works in a fairly legitimate way in Canada, unlike in the U.S. where practices like gerrymandering, voter suppression and 3rd-party suppression are rampant. Even so, while FPTP makes some limited sense in individual ridings, when you look across a region or country the results are clearly unfair. In the 1992, for example, the Progressive Conservatives got 16% of the vote across the country but won only two seats. The Quebec-only separatist party Bloc Quebecois, on the other hand, won 54 seats with just 13.5% of the vote, and Alberta's Reform party, got 52 seats with 18.7% of the vote.
Generally more flexible than old-fashioned proportional representation, MMP (Mixed-member proportional) represents the population far better than first-past-the-post.
MMP still defines single-winner electoral districts, and the electoral system for these districts could be decided somewhat independently of MMP itself. One assumes that to avoid "rocking the boat", these districts will keep using FPTP. However, MMP compensates for unfairness of the results by granting extra seats to parties that didn't get as many seats as they should given the amount of votes they got. Thus, your vote is far more likely to matter under MMP. It isn't my favorite system, but I would vote YES quite eagerly!
The NDP is backing MMP, and I would urge everyone to sign their petition about it.