When you browse and post a website using HTTP, then the information is transferred to and from your machine in readable text. So someone could read the info if they intercept it.
HTTPS means the data is encrypted while in transit, so if someone intercepts it, it looks like garbage and has to be cracked open.
So from a browser perspective, your information that you send is not secure, not that the website has been hacked, etc. Its not a big deal if someone intercepts your post and reads it, since they could just browse the forum and read it any way