X-Risks are any events that could potentially cause the extinction of all of Humanity, or even all life on Earth. X-Risks have happened in the past, and there is no reason to assume we are immune to them now. Rome seemed invulnerable for hundreds of years, and the Dinosaurs for millions, and both are now confined to the dustbin of archaeology and paleontology. The same could easily happen to modern society, and indeed the entire world.
While there are several possibilities for natural disasters to be X-Risks, SHfHS is dedicated to preventing the most likely X-Risks: those caused by humans.
Some of these scenarios sound like something out of Science Fiction, and so it is easy to dismiss them as flights of fancy. It is important to remember, however, that not only have there been planetary extinction events in the past, but that many of the predictions of Science Fiction have come true. After all, we live in a world where cyborgs walk around on artificial limbs, robots do our housework, and we all regularly carry around formerly fictional tools that contain all human knowledge and can instantly communicate with anyone, anywhere, at any time. We already live in the future. It's too late to be a luddite: these technologies are coming, or in many cases, are already here. We need to be able to deal with them safely, rationally, and humanely.
SHfHS is a small organization dedicated to raising money and awareness for existential risk reduction. SHfHS is a philanthropic foundation. This means that we don't directly do work on reducing X-Risk; rather, we fund people who do. You can help.