It means dividing people into two extremes

Most likely in the political context, where it means something that pushes people to extreme opposites. It is commonly used to refer to certain controversial topics, and as an effect on discourse caused by certain communication styles. When talking about topics it refers to a tendency to cluster at two poles of opinion. When talking about the effect, it refers to things that cause people to push toward those poles.

e.g. Trans rights are a polarised topic, because most people that participate in the conversation tend to be wholely for or against trans rights. It’s not usually a nuanced conversation, and , in my experience, the arguers are mostly talking past each other because they don’t have the same meaning when they use certain words.
In the other use, some of the ways people talk about trans rights are polarizing. When someone tries to use emotionally focused rhetoric to sway people away from uncertainty or inaction (Alex Jones: ‘They’re cutting our kids dicks off.’) it is pushing people toward un-nuanced poles of interpretation, meaning they have to fight the trans people or fight on behalf of the trans people who have just been maligned.

Why did…