If I submit a thread or a post and later decide I want it gone, how do I go about getting it deleted?

I just created a thread on this subject and then realized I may have answered my own question by clicking the delete button on the bottom right corner of the post on that thread. Doing so grayed out my post in the thread and the thread's title on the forum menu has [Soft Deleted] next to it. I have no idea what that actually did and if it deleted my thread or not because I can still see it if nothing else. (So if you see two of this topic, that is why.)

What does "soft delete" do and what does it mean if I want to get rid of one of my threads or posts permanently?

Soft delete removes the post from the eyes of everyone but yourself and the moderation team. It's basically there as a fail safe so people can't post terrible things and then immediately delete them (terrible in this context meaning against the CoC).

But yeah that thread you deleted is effectively gone now to the vast majority of the community.