What is distribution?

A distribution is a link between an Amazon S3 bucket (the origin server) and a domain name (which Amazon CloudFront assigns). You use this new domain name in place of standard Amazon S3 references

Amazon CloudFront is a web service to distribute content to end users with low latency, high data transfer speeds. To do that you create a distribution. This distribution identifies the S3 bucket or other origin server which have the original content. Each distribution has a unique domain name which can be used to request the pages.

When a client requests a page first time using that domain name, that pages is served to user from origin server. That page is also cached at the distribution. When the next time the request is made for the same page, that page is directly server by distribution.