You need to PAY for FREE things!!!

[Tichy]

The top one tracks "level"
The second one tracks how close you are to gaining a new "level"
The third one seems to track some statistic about your amount of daily activity.

I have no idea how the level thing relates to the military ranks for number of posts.
I also have no idea how being given good/bad reputation affects them.

You get some info by hovering over the three bars with the cursor but there's more info on your profile page. Click on your own user name. On the page that loads, there's a "visitor message" window. On the upper right there's a ">>". If you click on it, something called vbExperience will pop up for you to click which will give more details.

[Herode]

You'll find more information about how level is computed if you access your profile (by clicking on your own name), then go to the "VB experience tab" (not visible at first : click on the ">>" right of the rightmost tab, then choose the appropriate menu)