A content delivery network or content distribution network is a system of computers where your website is stored so it’s data can be served from multiple locations.
Let’s say you make a product called “product x” and you sell it through your one store front. No matter where your customers are located they have to come to this one store to get it.
So now, Wal-Mart sells your product. Now, people from all over do not have to go to one specific store to get product x and Wal-Mart is likely to be closer allowing them to get your product quicker. Since all the other Wal-Marts will be selling product x it will likely reduce the demand on your store.
So in this example, the product is like your website. Your visitors would be able to get the site served from multiple locations reducing the load on your server and possibly speeding up the load times of your website.
Why is a Content Delivery Network good for my website?
To illustrate the importance of a CDN using real numbers, one image being called from one of the data centers takes 302ms. That same image fetched for a user in Portland is 597ms, and from Washington DC takes 102ms. The worst case appears when coming from abroad – to fetch the same image for a visitor in Australia it takes on average 1,141ms for just that one image.