They are called 'stars' and show two things. The number of 'stars' is a visual indication of how many posts you have (If you're a regular user), each time you move up a post-count related rank you get an additional 'star'. The colour of the 'stars' represents your user class: grey for users, red for admins, green for premium members and blue for global mods.
It should also be noted that section mods get green "stars" in the sections that they have mod powers in but outside of that they just appear as regular members.
