A topic is a public forum post that starts a new subject. Topics are viewable by anyone in the forum and anyone can reply.
A message is like an email directly to one or server other members. Only those members can view and reply to the message. These are also called Personal Messages or Direct Messages on other forums.
When you click the “+ NEW TOPIC” button in the open green box on the Community home page you will get this:
If you enter a title, choose a category and enter the body text, then click the “+ CREATE TOPIC” in the dark green box, that will create a new public topic.
If you click the forward arrow at the top left, you will see this:
If you then click the envelop “New message”, it will change the interface over to creating a new private message. By its very nature, a new private message will require the entry of the usernames of one or more other members to whom you wish to message privately.
To learn more about how the forum works, please start a new message addressed to “discobot” with the title and body of “start new user”. That will trigger a short tutorial.