There are multiple categories in this forum, intended to help divide the discussions into relevant groups. While the forum software running here puts the posts in focus, contrary to how many forums do it, where you have to click on a category/board first to see the posts within by showing all discussions on the front page, this forum still have categories, and if you prefer, you can easily show the posts per category.
Just go to the Categories page (the link is also found in the main menu at the top [or hamburger menu if you are on mobile/small screen]) and you will see the categories, and from here you can click on a category to see just the posts from that category.
This page lists all the categories, with a small description of each, which should describe what it is intended for.
From this page, you can also follow a category by clicking on the cog icon, and also marking posts in that category read.
Following a category is used together with the view selector on the all Discussions page:
Basically, if you set the View to Following, you only see the posts from the categories you are following. You can use this to filter out categories you feel are not that interesting to you, or perhaps by having a view to what you are most interest in.
Note that some of the categories require special permissions. For example, you can't post at all in the Forum Rules & Guides Category, and you can only comment, but not start new threads in the Resources category.