An SLD (Second Level Domain) is the domain name that is located right before a TLD.
For example, in mozilla.org, mozilla is the second level domain of the .org TLD.
A domain name is not limited to a TLD and an SLD. Additional subdomains can be created in order to provide additional information about various functions of a server or to delimit areas under the same domain. For example, www is a commonly used subdomain to indicate the domain points to a web server.
As another example, in developer.mozilla.org, the developer subdomain is used to specify that the subdomain contains the developer section of the mozilla website.
Learn more
General knowledge
- SLD on Wikipedia