A domain is an identity on the Internet. In email, your domain is anything after the @ symbol. On the web, it's the part after www. So, pobox.com is our domain.
You can also get your own domain, that you can use for email, a website or both. That's a personal domain.
You can use a personal domain with your Pobox account. In fact, you can add a personal domain to your account, and become firstname.lastname@example.org, while still keeping your current @pobox.com address!