CloudFlare is a proxy, that makes web application faster and safer, as explained on its website:
https://www.cloudflare.com/
It has many servers all around the world and is sitting between the PGC servers and us, users, filtering legitimate traffic, preventing unwanted traffic, spam, dDsS attacks and other issues.
This question is very broad and hard to answer since I have no idea in what particular thing you are interested in. Also this is getting little bit off-topic, since it does not have anything to do with geocaching. It is just a technical solution used to server PGC. Please, try to limit your questions to the geocahing topic.
