Subdomains are an element of the domain name system (DNS). With respect to DNS a subdomain is essentially a domain which is part of another, larger domain. Strictly speaking every domain on the internet is really a subdomain, with the exception of the root domains. You’ll notice that many web hosting providers offer free use of subdomains for domains that you own, these can be used with both free and paid-for hosting plans.
Still, what is an example of a subdomain? It’s simple really. For example, docs.plesk.com is a subdomain of plesk.com. Likewise, plesk.com is a subdomain of the .com domain.
The use of subdomains and the use of domains in general go back to the era of ARPANET: the goal was to make it easier to refer to servers using memorable names instead of numerical sequences. Of course, ARPANET goes back a long way – it is a precursor to the modern internet as everyone knows it.
Subdomains are great for creating hierarchy. You can use a subdomain to organize the content on your website according to your chosen hierarchy, enabling you to separate key parts of your website. All you need to distinguish these parts is a unique subdomain.
Either way a subdomain is really helpful in keeping a clearer website structure that is easier to remember and easier to access.