Alexa is a ranking system of websites based on the amount of traffic that the website gets from those with the Alexa toolbar in their browser. It is not the most accurate way of telling how much traffic a site gets, but it is all there is.
Most Useful Resources:
Alexa Ranking of Podiatry Websites (PodiaPaedia)
How important is a good Alexa ranking? (ForumDr)
Do I need a good Alexa ranking? (Search Engine FAQ)
Alexa Rankings (Podiatry SEO)