Websites ranking shows you which one is more popular (more viewable by internet users)!
The ranking of these websites is based on Alexa's statistics.
Websites with a rank closer to the top (number 1) are so to speak more
popular than others because they seemingly have more traffic, meaning
more people view these websites, meaning they update frequently, meaning
you WANT to go visit those websites.
How does a website get its rank?
People download and install Alexa's toolbar.
The toolbar collects information from the user's browser and sends it to Alexa's servers.
There, this information is being stored, analyzed and categorized.
Thus granting every website a number which represents its location among others
in the world.
You can read more about Alexa's ranking here.