Fission or...

Imagine a large atomic nucleus (like that of uranium) as a slightly unbalanced billiard ball.

When a neutron is fired at it, the nucleus splits into two smaller pieces. This is fission. As it splits, it releases an enormous amount of energy in the form of heat, and also emits other neutrons that will split other nuclei in turn: this is called a chain reaction. This is exactly what happens in current nuclear power plants. A very small amount of matter (a few grams) is enough to produce as much electricity as a huge quantity of coal or gas.