You might want to know start by learning more about What is the Difference Between a Domain Name and a Website URL and what Domain numbers are, we suggest you to read our dedicated articles for that.
Understanding the Second-Level Domain
Here we have our example domain – example.com. A second-level domain (SLD) is the part of the website address or the domain name that comes before the top-level domain. In our case “example” is the SLD and “.com” is the top-level domain extension.
Subdomains, also known as child domains are an additional part of your main domain name, created to organize and navigate to different sections of your website. A quick example – store.example.com. – here the subdomain is “store”, the SLD is “example” and “.com” is the top-level domain extension.