Alexa Rank is depends on the traffic (how much visitors your website has), to increase the Alexa rank you need to increase the visibility of your website on search engine and on social media platform so that your website will get more traffic.
It is easy to manipulate Alexa rankings, but if you don't, it is one of many gauges to see where you stand against competitors. If you are looking to buy a site, do not use those numbers because as I said, they can be easily manipulated.
Alexa is a website that provides information about websites traffic stats and metrics, related links, reviews, contact information and etc. Although it can be manipulated easily, it has lost its importance and no one consider it because of that. Mostly people who are looking for purchasing the advertisements on website, they consider alexa ranking to take an overview on website performance in search engines.