A CNAME record is a redirect record or alias record that links a subdomain to an address.
Two CNAME records above point the subdomains to the example.com A record (i.e. the same webserver as the main domain). The last CNAME record points the notes subdomain to a different webserver (hosted on github).
It is better to link subdomains with CNAME records as if the main webserver changes, you just change A records IP address and all the CNAMEs will remain pointing to the same A record.