What is traffic rank???

[sumitkhare]

Im reading Alexa's "traffic rank" --- what is that???
For example,
A real estate brokerage website has a rank of: 3,471,370
Boston.com has a traffic rank of: 1,487
Does anyone know how to read this?
Thanks!!

[robindell10]

The main Alexa traffic rank is based on a value derived from these two quantities averaged over time so that the rank of a site reflects both the number of users who visit that site as well as the number of pages on the site viewed by those users.