A forum is simply another name for a place of discussion. Its principles are simple.
People can then enter the forum and, by clicking on the message, read it.
Those who read it can reply to the message by entering their responses in the message box underneath the initial posting. Because it is a forum and not electronic mail, the total context of all messages is preserved and easily searchable. This has the advantage over email, in that the messages remain catalogued under the initial postings, and are almost instanteously available to any member who has a web browser. Click on it. It will open and you will see two messages.