It just means you can host as many domains as you want--but you still have to pay for them, other than the one free one that comes with your account.
If you register it here, it would be $9.95/year for the registration.
You can register the domains where ever you want and just point the nameservers here. I have domains hosted here that are registered at Dreamhost, Godaddy, Yahoo, Namecheap, Registerfly, Directi, Name.com, etc...
Save up to $96 at Dreamhost with ALMOST97 promo code (I get $1).
Or save $97 with THEFULL97.