The forum is a place where people can discuss stuff without spamming the comment section of a post.
Creating a new thread
Click on "New" and you will be directed to the thread creation page.
The fields should be fairly self-explanatory.
Replying to a thread
Open a thread and click on "Reply".
You can also reply by quoting someone using the "Quote" command under a comment.
Threads can be placed into categories, but it's not required and by default the site displays all the threads in every category.
It's good etiquette to give the proper category to a thread, but if you forget and place a bug report in "General", it's not a bannable offense.
You can search the forum like you search posts, with the exception that instead of tags you search for words used in a comment.