It is a visualisation of your post number... Or the camera under it. Neuro has the most, while first posters have only one.
The forum Membership is broken into several member groups. Each member group has a name associated with it, and a number of bars, stars, etc.
As the number of posts goes up, a Member is automatically moved to a different member group, and the number of bars changes to the setting for that member group. We could have done it like Canon Cameras, and reduced the number of bard down to one for the highest level of posts.
One advantage of breaking the membership up into different groups is our ability to allow different forum capabilities to higher posting members. We can also setup member groups by invitation or subscription only, and make them visible to certain groups only.
That's why you don't see the groups reserved for selfies and nude photos posted by movie stars and celebrities
You don't see the user group that Admins use to exchange routine forum operation information either.
The Buy and Sell group is another example of a limited membership group, everyone can see it and post to a existing topic, but starting a new for sale topic is limited.