Fusion energy, simply, is the exact opposite of fission energy, which comes from splitting an atom and is widely used to power nuclear plants and weapons. Fusion occurs constantly on our sun, which produces most of its energy via the nuclear fusion of hydrogen into helium. When nuclei fuse, they create a heavier nucleus and produce a little leftover energy in the process.

Fusion doesn't produce runaway chain reactions the way fission can, so there's no need to worry about meltdowns. Nor do fusion reactions produce the large amounts of dangerous radioactive waste that fission reactions do. That's why it's such a dreamy source of energy.