The "Trial period" does refer to advertisements. After it expires some ads will appear on your forum, but you have the option of paying for them to be removed.
Your forum will not be deleted as long as visits to the forum are made. You will be given notice by email should your forum be scheduled for deletion. Do check your spam folder or white list addresses ending with nabble.com to ensure you see any warning emails.
Volunteer Helper - but recommending that users move off the platform!
Once the admin for GregHelp now deleted.