It is an alternate domain name provided in AWS.
CNAME links your file instead of using the domain name that cloudfront uses for your distribution.
You can read more about CNAME and REC here:
https://www.edureka.co/community/23415/difference-between-a-rec-%26-cname-in-route53
for more details you can refer to the documentation from AWS:
https://docs.aws.amazon.com/AmazonCloudFront/latest/DeveloperGuide/CNAMEs.html