A CDN speeds up data delivery from the web server to the end users, by distributing data across the globe. Content can be any static content, from images, to videos or even large downloads.
The content will always be served from the location closest to the user, meaning they get exceptionally fast connectivity wherever they are in the world. What's more, due to its distributed nature, a CDN has no single point of failure - if any one global location is down, the next closest will be able to serve the data.