Okay, so if you hover over the bar it gives more information.
For your first 10 posts you are a Forumini Lurker (Forumini Midshipman) and the bar goes to the right to indicate progress to your 10th post. For posts 11 to 500, you are a Junior Forminian (Forumini Ensign) and it keeps track of progress the same way each rank. For posts 501 to 1,000, you are a Senior Foruminian (Forumini Lieutenant). For posts 1,001 to , you are a Forumini Captain. Less is known about this rank because nobody is there yet. redoctober, brigs, and I are the only members with 500+ posts, and it's probably not coincidence that we are active online game players.