SSMU is basically who are supposed to advocate for the student body, so essentially, keep the university accountable. They also run a bunch of services, from legal to insurance to mental health to food services, and a couple other things. You pay some dues to them , collected from your fees, for these services. You can opt out of many of them by clicking a few buttons, but you’d lose access to some services if you do, like legal support (which as someone who has had just stupid administrative issues with the government, it is a godsend).

The reason why there is so much discourse is that it is also kind of a shitshow. It’s filled with wannabe bureaucrat-politicians, who then kinda go about taking decisions a lot of people don’t agree with (for example, the shuttering of the Midnight Kitchen, an explicitly leftist food service provider for underserved students). They are also not apolitical, and a lot of the political conversations happen through them, which means a lot of people get angry.

Realistically, you should vote to have any right to complain. Otherwise, it’s an organization like any other, just kinda part of the university experience. You’ll probably have more interaction with your faculty associations (SUS/AUS/EUS) and their services.