MMP is just New Zealand's way of making Parliament look a bit more like the country. Two votes, one Parliament, a lot of coalition snacks.
Every time an election rolls around, an adult somewhere goes "remember, your party vote is the important one" and then doesn't explain why. So here's why.
MMP stands for Mixed Member Proportional. The "mixed" bit is the giveaway - you get two votes, not one.
Vote number one is your electorate vote. That picks the local MP for the patch of the country you live in. One person, one seat, whoever gets the most wins. Pretty simple.
Vote number two is your party vote. This is the secret boss of the election. It decides how many of the 120 seats in Parliament each party gets overall. If a party gets 30% of the party vote, they get about 30% of the seats. That's the "proportional" bit.
We didn't always do it this way. Before 1996 we used a system called First Past the Post, and small parties basically got nothing - you could win 200,000 votes across the country and end up with zero MPs. People got cross about it, we had a referendum, and MMP won. So now smaller parties actually show up.
There's one gate to get through: a party needs 5% of the party vote OR to win at least one electorate. Otherwise they don't get in. It stops Parliament filling up with tiny one-issue parties, but it's also the thing election-night nerds (hi) stay up late watching.
The other big thing MMP does is force coalitions. Almost no party ever wins enough seats on their own, so the biggest party has to make friends with smaller parties to form a government. Grown-ups complain about this a lot. I actually think it's the point - it means more than one group has to agree before anything big happens.
So here's the kid-sized takeaway: your electorate vote picks a person, your party vote picks the shape of the whole Parliament. The party vote is doing the heavy lifting, and a surprising number of grown-ups still don't fully realise that.
If you want to play with it, the Vote in the Zoo lab on the site lets you mess with party votes and see what coalitions fall out. Way more fun than it sounds.