Website and email addresses have multiple parts.
For example there are typically 3 parts to a website address: www.mywebsite.com/webpage.html. The middle part is the domain name - mywebsite.com in this case.
Email addresses have 2 parts - firstname.lastname@example.org. The domain is the last part, again mywebsite.com.
If you are asked to enter your email domain, you should enter the information after the @ symbol.