To understand what is a subdomain let's dissect a domain name from the top down. A complete domain name includes a top-level domain or TLD and a second-level domain or SLD. the TLD is the part that occurs right after the last . in the domain name. This is why tlds are sometimes called domain extensions in this example . com is the TLD while there are several types of TLDS the most common are dot com that org and dot net.
The SLD or second-level domain is the part that occurs immediately in front of the last . in the domain name in this example it's the name of the business. The SLD is what makes the domain name unique. Now this part right here that displays before the first . in the domain name it's the sub domain. That’s right fun fact www is actually a sub-domain.
The good news is you do not have to use www for your sub domain instead you can replace it with almost any word. To create a subdomain with a unique web address without having to purchase a new domain name. Subdomains function as an extension of a registered domain name allowing you to send visitors to a different web address or allowing you to point to specific IP addresses or directories in your hosting account.
The most common use for sub domain is organizing or dividing web content into distinct sections. If your website has a substantial amount of content on a given topic or there's content on your website that you want to draw attention to people to see for marketing purposes you can create a subdomain for it.
A sub-domain sets this content apart from the main site and gives it a unique web address without having to register a new domain name. Consider the dog friendly winery Molly's bark and whine during her first year of business the owner has amassed a large photo collection of dogs who visited the winery on her website to highlight the businesses unique dog friendly atmosphere and provide a dedicated space for her expanding photo gallery. She creates the sub domain dog under her registered domain name Molly's bark and whine com. the www in her domain name is replaced with the word dogs and a new web address for the dedicated photo gallery is dogs . Molly's bark and whine . com as an added bonus this sub domain is treated as a unique web address and in turn may drive new customers to her website
Subdomains also offer a great way to create language specific versions of a website or to accommodate information that may not be critical to the primary website such as an events page, blog or store.
To recap a sub-domain as a subset of a registered domain name sub domains are most commonly used to divide websites in two distinct sections that have unique web addresses

Long story short,
www.mydomain.com (domain/like a house)
blog.mydomain.com (subdomain/a room in the above house)