For a forum with no sensitive data, It's not a huge problem. Communications between you and the server and vice versa are unencrypted: that is to say that were they to be intercepted, they would be shown as plain text as entered in forms (like replies or passwords), and addresses requested (hyperlinks clicked).
In a forum that shares knowledge and discussion with no real commercial value, its probably ok, but do bear in mind that anyone with access to any of the steps between you and the forum server can read it.
Obviously, don't use the password anywhere else, don't share sensitive personal or security information (even in private messages), and you'll be fine.
https://www.cloudflare.com/en-gb/learning/ssl/why-is-http-not-secure/]
[url]https://www.cloudflare.com/en-gb/learning/ssl/why-is-http-not-secure/ [/url]