What is Content Delivery Network and what does it do?
A content delivery network (CDN) refers to a geographically distributed group of servers which work together to provide fast delivery of Internet content.
A properly configured CDN may also help protect websites against some common malicious attacks, such as Distributed Denial of Service (DDOS) attacks.
Source: What is a CDN? -Cloudflare
There are quite a few popular CDN:
Cloudflare, Sucrui, KeyCDN