Alexa measures the traffic on your website and provides a numerical rank to your domain. The rank helps to denote the popularity of your domain in search engines. The Alexa rank can be improved by the good content update and also by building backlinks from other relevant web pages.
Alexa rank is the measurement of your website performance over internet and how popular is your website on the basis of traffic your website is getting, so if your alexa is good then it means your website is popular and getting traffic.
I heard that Alexa rank has not significant importance at this time. Usually I look at DR, PR, number of external/internal links of the site, amount of traffic and check Web Archive history.
Alexa rank is very important for SEO purpose, your website's value and impression depends on your Alexa ranking. In SEO mostly webmasters watch and decide your site's valuation by it's Alexa ranking.