What is a Subdomain?
A subdomain is a part of a root domain. For example, if your website is bobs-tires.com, a subdomain would be calendar.bobs-tires.com or mail.bobs-tires.com.
How do subdomains benefit my website?
Creating subdomains makes it easier for your visitors to navigate your site or find what they are looking in search engines. Since subdomains can be seen as unique domain names by search engines, you can tailor your SEO terms on your subdomains to be pertinent to what is on that specific page rather than what the root domain's SEO terms say. Additionally, when you are creating separate pages on your website, you can create bobs-tires.com/mail and then point the subdomain mail.bobs-tires.com to the same page. This way, if someone looking to send you mail types in either address, they will find exactly what they are looking for.