A domain name is the root name of your website.
It is the word(s) that come after the https:// or https://www.
An example of a domain is forthdev.com (from https://forthdev.com)
The domain name is stored in an online library called a Domain Name Service (DNS) server, along with the IP address of the server which hosts your public directory.
When someone tries to visit your domain via their browser, the DNS server will provide them with the corresponding IP address. They can then connect to this location directly, and download and render your website files in their browser.