How does a loan work that says it is a 360 month term but requires balance payed in full at 15 years?

I was looking through my loan documents and it says that my 2nd mortgage, which was part of the original loan given to avoid PMI, is a 360 month loan but after further research finding out that I have to pay any balance that is there at the 15 year mark. How does that work I am very confused about that? Thanks
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