Your 'domain' is your unique identifier on the Internet.
You will have registered a domain - "yourhotel.com" for example - from a large vendor like GoDaddy, 123-reg, Ionos (formerly 1&1) or from a smaller reseller, possibly as part of a previous website design package.
The domain has an annual "hosting" or registration fee from the body who administer the domains to ensure it remains under your ownership. In the UK we have Nominet for example who manage all domains ending in ".uk", but you would register the domain through a vendor like 123-reg, who will invoice you each year (or every 2 years) for managing the registration & renewal. This fee is independent of any associated services that may be provided, but can sometimes be bundled by a vendor as a part of those other services.
Having a domain then enables you to direct any visitors to the server on the internet which is hosting your website. Your Domain Name Service (DNS) record is set to associate "www.yourhotel.com" with the web page code and images that make up your website.
If we are providing you with a website, you will need to give us access to your domain login (ie GoDaddy, 123-reg etc) so that we can create a DNS record to point at the website. Configuring the DNS record for you is a part of the website package.
You can sign up with an email service provider who is completely independent of 1. Your domain registration company and 2. Your website design / hosting service. It is equally common to have all 3 services with different providers, or the same provider.
If you are using Caterbook to send emails to your guests, you are likely to be using our email servers to send "on behalf of" the email service you have signed up for.
Many email service providers these days use Sender Policy Framework (SPF) to reduce the volume of spam emails being received. To reduce the possbility of emails sent from Caterbook being tagged as spam (and therefore not being delivered to the guest), you may need to create a DNS entry in your domain login to add our mail servers to your SPF record.