Maybe you haven't used the search form in this forum, because it's almost impossible not to find nothing about this. But I'll explain it to you for more convenience:
When you apply for your free hosting on trap17.com you can either choose a subdomain (name.trap17.com) or a domain (name.com). The subdomain is free, but it's a little longer and if you want to start a business, a community or whatsoever you should buy a domain name.
There are a lot of hosting sites that allows domain registrations on the Internet. You could use google to search for this. Some sites offer you only the domain name (name.com) for about 5$/year, that means that the domain chosen is your property, registered on your own name.
After you paid for that domain, it should pass about 1 day to be fully functional. Then you'll login in the members area with the details that the site where you bought the domain gave you and you'll see that you can change DNS settings.
Go there and enter the DNS settings of trap17.com in case you chose that domain name when you applied for your free hosting in this forum. You should wait about 1 day more and you'll see that you can access your domain. Now you can upload the files of your site and get your businness started.
Why do you need to signup with trap17 hosting and enter the DNS details?
Because you only bought the domain name (name.com), but they haven't offered you any hosting space, only that name.
I think that I covered most part of the need of entering the DNS nameservers. If you have any other questions about this, I'm ready to answer you.
Comment/Reply (w/o sign-up)