This is a fairly new feature, and one that questions have been asked of a few times, so I'll attempt to explain it.
Thanks to Paddy for the donor post
The green bars are an indication of a users reputation. Similar to 'Likes' facility, reputation is gained (or lost) by clicking the star highlighted at the bottom of each post. A positive repuation can just be left, a negative reputation involves leaving a comment.
You can see where your reputation has come from by clicking on 'Settings' in the top right of your screen.