Domain name service or DNS is a system designed to make accessing servers easier by using easy to remember names as opposed to long IP addresses. You use DNS every day while surfing the web. For instance, yahoo.com and google.com are examples of "domain names." All computers connected to the internet have an IP Address in order for the computer to find the defined location. DNS translates IP address numbers into names for human usability, then back into numbers for computer usability.
Domain names are registered to an individual or a group through a registrar. There are many registrars such as godaddy, twocows, etc, who register domains for a fee. Each domain name is pointed to a public facing IP address. For example, 18.104.22.168 is an ip address to which yahoo.com points.
As you register your domain, note that the information used to identify your organization will ultimately be used to compare the information that is sent with your SSL Certificate request. This domain control validation process verifies your identity.
Some domain registrars also offer email accounts using the domain which you are registering. As part of the Signed SSL Certificate validation process, you may be required to respond to an email sent to an account at the domain for which the SSL certificate is purchased. You may wish to add an email account with your domain purchase that will identify an admin, postmaster, or webmaster. For example, an email may be sent to an email address such as firstname.lastname@example.org to verify that you are the owner of domain.com.
Once you have registered your domain name (which is unique to you or your organization), you may set up through the DNS what IP address to which you would like your domain to "resolve." When obtaining signed certificates through a CA (Certificate Authority), they will require you to provide them with a domain name and evidence that you are the owner of this domain. This evidence usually consists of sending an email to an email address on that domain. Having a registered domain that is properly resolving to the clients IP address will need to be completed before obtaining and installing a trusted and signed certificate.