Alexa.com offers a search engine, a directory and a toolbar (among other services). It also has a resource called Alexa Traffic Rankings (click the traffic rankings tab at the top when you get to Alexa.com), which is what we are focusing on when we study AlexaRank. You can input any domain name into the traffic rankings form and Alexa will spit out the current ranking data for that site.
The AlexaRank blue bar is a snapshot representation of the full Alexa traffic rankings, which include graphs that track the history of practically every website online, or at least every site Alexa “touches” or is exposed to via its toolbar. Each site has a ranking number, the lower the number the better, which is an aggregate of the last three months of traffic data.
I’m glad to have the benefit of AlexaRank and I openly admit I pay it too much attention, just like PageRank. It’s a handy tool for evaluating a website and calculating a VERY rough estimation of its traffic. It should be considered a “trend” statistic as opposed to anything concrete and works well in that role because it aggregates data over a rolling three month period.