It depends who you ask. Alexa represents the popularity of your site. the lower the number the better. if you have a 500,000 Alexa ranking that means you are the 500,000th most popular website in the world.

The problem with Alexa is that it can be manipulated very easily. I have seen podcast websites out there that get no viewers but some how the Alexa is a 100,000. People can go and buy website hits or Alexa rankings from companies. If google feels you are manipulating your search rank you can be banned and never be seen again (on google).

The best way to increases your Alexa is by going to Alexa.com and registering your website. They will determine your rank based on how many website uniques / pageviews your website gets.