A domain is the name of a website. For Example, website address is www.example.com. "example" is the domain name of the website.
It's a unique address on the internet. Just as every house in your street has its own street address, every website on the internet has a unique web address, known as a domain name.
You can have multiple domain names pointing to the same website or you can have one domain name with many websites on it. The first thing you would do when starting a new business or even starting a blog would be to find a domain name that no-one else has.