There's a link to my subscriptions but it says I'm not subbed to any posts. How do I sub to something and what does that do?

Where do you see that link??

You can see the forum thread subs from the link at the top right (sub-menu) or if you're on a mobile browser, expand the sub-menu.

As for what it does and how you use it, it's not yet ready. But when it is, you'll automatically be subbed to threads that you posted in and you'll be able to track them in there.