"This joining is called nuclear fusion." From the passage, "fusion" means

In the time it takes to blink, the Sun gives out far more energy than all the people on the Earth use in a whole year. Without it there would be no light, no warmth and no life. So where does all that energy come from?

The short answer is that the Sun is enormous. It is made mostly of hydrogen, like the planet Jupiter, but it is vastly bigger: it would take almost a thousand Jupiters to fill it. All that material is held together by an enormous amount of gravity, and gravity squeezes the centre of the Sun with tremendous pressure.

At the centre, or core, the pressure is so intense and the hydrogen so tightly packed that its atoms crash together hard enough to join into a new element, helium. This joining is called nuclear fusion, and it releases energy. Each reaction helps the next one happen, so the process goes on and on.

The energy builds up until the core reaches about fifteen million degrees Celsius. From there it works its way outwards, layer by layer, until it reaches the Sun's visible surface and pours out into space as light and heat. A small share of it, after a journey of about 150 million kilometres, is the sunshine on your face.