Domain is the human-readable web address people use to find a website.
It matters because development decisions affect speed, security, scalability, tracking, SEO, and maintenance. A site that looks good but is fragile or hard to update can quietly hold back every marketing channel.
Example
A company registers example.com and uses www.example.com for its website. Domain registration gives it control of the name for the registration period; hosting and email still need their own service configuration.