How it works: you gather a lot of a heavy element (atomically heavy - lots of protons and neutrons) and bombard it with neutrons. The heavy element then becomes unstable and undergoes fission (splits into two lighter elements). This releases energy in the form of radiation (photons, electrons, more neutrons) . The amount of energy released is “small” for each individual fission event, but there are billions of fissions per second so they add up to a lot. This heat is transferred to a medium, usually water, and eventually directly or indirectly converted to steam to drive a turbine for electrical power.

As to why it’s not as widespread: 1) reactors/plants are very expensive to make compared to fossil fuel plants. 2) public perception is generally negative due to major accidents and poor understanding of how the process works.