What are the numbers before some Apple Forums?

That generally means that the poster has looked at a kb (knowledge base) article from Apple and didn't find his/her answer there. There is a link to the communities on most of these articles. If you wanted to, you could try a search for the article number on the Apple site.

The HT prefix is for a Help article, while the TS prefix refers to a Tech support article.

That is the code number of the Apple article from where the poster of the question first went in research of their problem and started the thread at the bottom of the page with "Start a Discussion".

Just go to one of those discussions and you'll see at the top where it originated and you can click on the link.