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.