There are many domain extensions out there, a .tk domain can be free or additional services carry a cost.

Essentially the .tk owners decided to focus their service on the pitch that you can

transform any long url into a short .tk domain

Essentially they just make it easy, you put your long URL in and they provide you with a shorter .tk URL. You can then send anyone to your new .tk URL and they will be forwarded to the longer URL you put in. This can also be accomplished by purchasing any domain and forwarding to the longer URL. However .tk makes it free, or you can purchase additional services for a fee. Eventually if there is not enough traffic you will lose the free .tk domain unless you upgrade to a paying account.

See the .tk site here.