Subdomain
A subdomain is an extra name placed in front of your domain, like shop.yourshop.com.
In a simple example
Your main site is yourshop.com, and your booking page lives at book.yourshop.com. Both belong to you.
Why it matters to you
You can give one part of your business its own address without buying another domain.
Do not confuse with
A subdomain is not a new domain you pay for. It is a free extra name under the one you own.