The company whose computers every fifth click on the internet passes through - the security at the door that most people have never heard of.
In practice Cloudflare works as a doorman in front of half the internet. When you open a site that uses it, you actually enter one of their computers first - it decides whether to let you in, where to serve you the page from fastest, and whether you're even a human or a machine sent to break the site. The site owner doesn't touch any of this. His traffic just passes through them, while they guard the threshold.
Their main job is defense against something called a DDoS attack - literally burying a site under millions of fake visits at once until it collapses under the weight, the way a line in front of a door can block an entire building. Cloudflare filters that line before it even reaches the site's door. They also keep copies of pages all over the world, so that every one of your requests doesn't have to travel all the way to the server - the computer where the site actually lives - somewhere on the other side of the world.
They exist because the internet was built without a lock on the front door. Every site, big or small, is exposed to attack from anywhere in the world, and most owners have neither the money nor the people to run their own round-the-clock security. Cloudflare sells exactly that - ready-made security, switched on with one setting.
Their clients range from one person's blog to banks and government sites. That's exactly why, when Cloudflare has a technical problem, the internet notices - not because their site is important, but because every fifth click on the world's network passes through it.
I'll Say It Like a Curator
What you can't see from the outside is how concentrated the power behind this security is. When Cloudflare's servers go down for a few minutes, it's not one site that falls - a noticeable chunk of the internet falls at once. It's convenient for site owners. Less convenient once you realize how much of the network depends on a doorman whose door you never get to look behind.