It comes from you. Every registered member of a site can add content. Most of it will be discussion in the forums, some will be files uploaded for sharing, and some may be special content such as posting organization bylaws (if applicable to a particular site), conducting polls, or perhaps a blog with a good story, a joke, or a photo of events.

Creating content 

So how do you do it? Look at the left column for CREATE CONTENT, click on that, and you'll see the types of content you are authorized to create.  

Click on one. You'll see some fields and the ones with a red * are required. Fill in what you think fits. Every type of content has the main content area where you simply type what's on your mind.  

Once at the bottom you'll be forced to preview it and you'll have an opportunity to make changes, or just forget it. When you have it the way you like, click SUBMIT.

It's not "posted" until you click SUBMIT. That's all there is to it.

About comments

To each item of content you can add a comment.  You'll see the comment capability below each item. Look below where I have created a comment to my own item here.

You can even comment on comments, known as a reply.