CDN stands for Content Delivery Network.
A CDN is a cluster or group of servers around the world that host content (images, media, files). That when called on will deliver a request (image) to a user. Spreading the servers around the globe serves two purposes: One is to enable having low latency and quicker response times by having the request not bounce around the globe and though many datacenters before coming back to the user. Two less chance of outages or down time if there are many servers available to fetch a resource from.
Effectively a CDN speeds up and spreads loads for requests made to your website.