What is a domain name?
A domain name is the name of the website that you visit. For example, this website is FAQ.EranBucai.com.
Here is a great simple and short explainer video from YouTube from a company called GoDaddy, that sells domain names.
It is important for me to note that while I seriously appreciate GoDaddy's high quality explainer video, I have been a customer of GoDaddy and I definitely do not recommend buying a domain name from them.
The company I recommend buying a domain name from is called SiteGround.