Hi Karen- Nice to have you here.
A thread is a whole new line of conversation. A post is a reply or a comment you add to an ongoing conversation (thread). So threads groups individuals posts together.
You have just started a new thread, which is great because you aren't directly addressing something you saw in another thread. If you want to reply to something you see in someone else's post or make a comment about that same topic, you can add a new post (by clicking on the new reply button).
I'm adding my own post to the thread you started with your own post just a few minutes ago.
I hope I'm making sense. I'm not too great at explaining things sometimes.