It is a formula determined by your race finishes, among a few other factors. Without going into the actual math, the better you finish the higher your Rank. The Rank scale is 0-10 (though it is possible in very extreme cases to have a rank edging below 0 or above 10). A word of advice: Try your very best to finish races short of driving a damaged unsafe car on track with your competitors. DNFs will drag down your ranking.
A realistic rank for a superb driver with a high percentage of wins and poles, is an 8.
You will need to have a certain number of events in your history before your Rank is included on the main Rankings page. Currently, that number is 9 events.