Cryptocurrency mining, similar to Bitcoin, much to complicated to go into the specifics but lets just say it uses your CPU power to do stuff, think SETI@home, except instead of deciding whether or not you want to do it just going to a website launches it via java, without letting you know, using your computer as long as the website is opened without permission. Basically they make money just because your computer is on their site.
Luckily my anti-virus software also checks webpages and aborted the connection, but it seems odd they would have it. I mean they aren't one of those places that gives free content but needs to keep the lights running, they are by default making money by selling things.