a way by which the traffic of a website is known

These is an standard metric of the traffic rank position in the country vice and all over world also,it give the rank on past three months traffic report..

Alexa
 

Alexa rank is your website rank around the world based on your traffic and updates you are giving on the website.

Alexa ranks you by the amount of traffic your site gets visavis your competitors.

Alexa measures the rate of guests to a specific site. Alexa can gauge just a little rate of web clients, and these are the ones who have Alexa toolbar introduced. So the higher the site activity to a site, the higher its rank is.

Alexa's Traffic Ranks are based on the traffic data provided by users in Alexa's global data panel over a rolling 3 month period. ... A site's ranking is based on a combined measure of Unique Visitors and Pageviews. Unique Visitors are determined by the number of unique Alexa users who visit a site on a given day

Alexa.com is a website that ranks websites based on traffic, so in terms of SEO, moving up or down the ranks of Alexa is indicative of how effective your SEO campaign has been.