Among your switches at Admin Panel -> Settings -> Switches you have an option to turn on a popularity rank. This used to be the google pagerank of the site, but google no longer makes that information accessible to the public, so it's now a similar 1-10 popularity bar image based on alexa's popularity rank for the domain name.

If enabled, the popularity rank will be calculated at the time of article submission. Preexisting articles will be slowly updated at timed intervals (one approximately every 10 page loads) in order to prevent traffic spikes that could cause alexa to ban you. You can get ranks updated quickly by regenerating them, but this could cause alexa to ban you, so patience is recommended.

A sort option will automatically appear to sort by domain popularity, and a small bar will show up next to links in the articles indicating the popularity. The bar images come from the images_yourthemename/pagerank/ subdirectory.

If you'd like to display the popularity rank as a number (1-10, or 0 for unranked), use {LINKALEXAPOPNUMBER}. If you'd like to display alexa's estimation of the domain's popularity rank, where 1 is the internet's most popular site and less popular sites can number into the millions, use {LINKALEXARANK}. For the bar image, use {LINKALEXARANKIMAGE}.