The domain is the part that comes after the @ in an email address. Free email providers get to slap their own domain on your email (e.g., yourname@yahoo.com), which isn’t the most professional look. If you’d like a custom domain in your email setup, you have to register a domain name with a domain registrar (e.g., Hostinger, SiteGround, InMotion Hosting), and then purchase email hosting to go with it. Since domains are public property, you have to renew them by paying a fixed amount every year – it’s a lease rather than a purchase. If you don’t renew your domain, you’ll lose rights to it and someone else can buy it, which means you won’t be able to use your custom email address.