MMP stands for mixed member proportional. There is a referendum being held in Ontario on the provincial election day concerning whether to switch to this new system or not.
There are 107 electoral districts in Ontario now and each one of them has a representing official. They can be affiliated with any party and are elected by the voters within that district, or riding. The candidate with the most votes takes the entire riding. That is the "first past the post" system we have now.
Proportional government would see voters casting votes for a party instead of a person. The party would generate a list of candidates with their important figures at the top. The percentage of votes for each party would directly correspond to the percentage of seats they would get in parliament.
MMP is a mixed system that would utilise 90 fptp seats and 39 proportional, or list, seats. The voters would cast two ballots, one for their local member of provincial parliament and one for a party. The winner would take the riding and assume their assigned place in the fptp seats. The list ballots would be counted province-wide and the number of list seats for each party would be assigned.
That is basically MMP.
The Citizen's Assembly for Electoral Reform's website.
The official information page for the referendum.