This tool helps you to find out the alexa rank of any website or domain. Alexa rank is a ranking of the websites in terms of number of visitors they receive. So, lower the alexa rank, more popular a website is, e.g. a website having rank 1, is considered to be the most popular website.
Also, alexa rank is only an estimated rank, it is not entirely accurate all the time. But usually more popular a website is, more accurate is its alexa rank.