If you already have a website, you probably already own a domain and are likely using that domain with that website. For example, you might have a blog titled "My Awesome Blog" which can be found using your domain: http://www.myawesomeblog.com
If, however; you have your podcast and don't already have a website, you can purchase a domain at a domain registrar (such as Pair Domains) and add that domain to your Libsyn show. This will allow you to use that custom domain with your Libsyn RSS feed and Libsyn-generated Podcast Website.
There is a $2 per month fee for hosting a custom domain with Libsyn, but this fee will be waived if you purchase your domain through Pair Domains.
You can set this up by accessing your account through Libsyn 5. See Set Up Your Libsyn Podcast Website. For help, contact our support staff at support@libsyn.com.