for delivering internet content
from a content
provider to end users
Your browser only has to go to a (hopefully) nearby site to get the stuff, instead of fighting through the myriad Internet bottlenecks.
Some CDNs operate as hosting services, and they do a strict replication of content to a fixed set of servers. Others use a more dynamic approach: all end user traffic is directed to the CDN; content is srved locally if possible, but fetched from origin servers when necessary.