A subdomain is the subset of a domain with which the domain can be divided into distinct sections. For instance, a website "abc.com" can be divided into "english.abc.com","french.abc.com" etc. on the basis of languages. These subdomains act as separate web addresses for the users and the users can access the main domain "abc.com" in these languages. Subdomains are a great way to segregate your data into different sections.
In the case of Content Delivery Network, subdomains can be created for the main domain and the content can be distributed as per relevance. The CDN servers are distributed over a wide range of network so that the content can be delivered across multiple regions effectively and swiftly. If the content is also divided into subdomains, the users aiming for a particular section of the domain will visit the subdomain pertaining to it.