The Alexa Rank gives any website a region or nationwide ranking. The lower the Alexa Rank that a website has, the higher the popularity of it.
To measure this, Alexa takes into account the number of pages that are looked at on a website and the number of visit that each website has.
An easy way to look at it is to think of Alexa Ranking as a sporting league table, with positions starting at 1, all the way into the millions.
Although it does give an insight into how popular your website is, the data collected to compile these rankings are from users that have the Alexa toolbar installed. This would mean that rankings higher than 100,000 may not be reliable and should only be used as a guide.
Having said this, Alexa does also show if a given website is popular in a particular country or geographical area. This has been shown to have advantages as it allows companies to see what area in which they are searched for the most.