A domain name is used to give your website an address or URL so that others can access it. There are different levels of domain names - the two most common ones being a top-level domain or a subdomain.
A top-level domain is a name that identifies a website or websites on the Internet. These names appear as a part of a Web site's URL (Wikipedia). For example, in www.yola.com, "yola" is the domain name. To have a top-level domain, you have the option of purchasing this through Yola. Here's a tutorial on how to do this: Buying your own domain from Yola.
A subdomain is a domain that is part of a larger domain (Wikipedia). Yola offers free subdomains which would look like this: http://www.yoursite.yolasite.com. If you are interested in going this route, please read this tutorial: Publish to a Free Subdomain.