The domain is a name that, along with a TLD ( Top Level Domain - Eg. .COM; .PT), identifies and allows you to access your project online. This allows you to easily memorize the address to your website and / or your email accounts.
Example: webhs.pt
Domain is not web hosting
The concept of domain is often confused with web hosting. However, they are independent services that normally complement each other.
The web hosting is performed on a specific server (web or HTTP server) where the contents of that website / project are stored and the same contents are made available to the devices that request them, through a specific communication protocol, the HTTP / HTTPS protocol.
This process is independent of domain names and access to the contents can be done by indicating only the IP address of this web server, however access to these contents through a domain makes it much easier to memorize / access.