It's an invite from which you create an account and after you go through all the explanations, you get a hashlet. It gives a little more than a faucet, but as long as it's free, you can't complain.
The deal is that I think that the guy that invites you will also get one more hashlet, that's why he tries so hard to convince you
Update: I seem to also have the option to send such invites, whoever needs one, PM me with subject "hashlet" and the e-mail where to send invite.
This is the best thing about this website ! you're not oblige to buy any ghs in order to withdraw your earning ^^