A public key is used to create a DNS record. That same key is also used to digitally sign the header of emails that are sent. When the recipient's provider receives the email, they check the sender's DNS records and the sender's authenticity is validated by the matching key. The message is then delivered with confidence that the sender is who they claim to be.
These instructions require you to create a CNAME entry in your DNS records. If you need assistance with this step, we recommend you contact your DNS provider for assistance because the steps may vary depending on your provider. Below are links to help articles of several common DNS providers to help you.