Fission reactions are the kind of nuclear reaction we use for nuclear power plants and nuclear submarines, and also to set off a nuclear bomb. With fission reactions, you take very large atoms like uranium and plutonium and smash them apart into smaller atoms. Because it takes a lot of strong nuclear force to glue these very large atoms together, when they're smashed into smaller atoms those atoms don't need as much energy to hold them together, and the extra energy escapes in the form of photons - light and heat - and also neutrons. Unlike fusion, fission is not very common in nature, though sometimes very large atoms do break apart on their own.
Breaking apart these very large atoms also releases a lot of radiation. If the radiation gets into the air, it can kill people or make them sick. Some people get sick right away, some only get sick months or years later. The radiation is very strong for a few hours, and then quickly gets weaker. People can still get cancer from being near where a nuclear bomb went off even two years later, but the effect doesn't last forever.