If you try to visit a quarantined sub, you get a splash page asking you to verify because the sub is a no good very bad place full of wrongthink. Then if you click "yes", you can see its content, but all the pages show a bright yellow "quarantined" logo, with a big button saying, "leave the quarantine" to exit the sub and save your precious mind from the wrongthink.

TL;DR It's basically a "soft" ban. People can still access a sub, but they're mildly harassed for doing so.