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.