If you bang the nuclei of two atoms together hard enough, they can fuse together to create a different element. If you use the right elements, the resulting fused atom will weigh less than the two you've banged together to form it, and the difference in mass will be released as energy, as predicted by Einstein's famous E=MC2 equation. C2 – the square of the speed of light – is a rather large number, so a small mass of fuel can produce a large release of energy.

The problem is, atomic nuclei are extremely tiny – and positively charged, so they repel one another, making it extremely hard to bang them together in the first place. It happens all day long in the Sun, producing the energy that warms the solar system, but then the Sun's got colossal gravity as well as super-high temperatures on its side, respectively pulling atoms toward the middle of its core, and making them vibrate with such ferocious energy that they randomly strike one another and fuse together, releasing yet more heat in a chain reaction that won't end for billions of years.