The Sun, like many other stars, is a huge ball of ionized plasma, mostly hydrogen, that is fusing that hydrogen into helium in its core.
The process requires incredible temperatures (15 million K) and immense pressure, but it also releases an extraordinary amount of energy. Every second, the Sun releases more energy than all of the power used on Earth for the past million years. Only a tiny fraction of this energy reaches Earth, and the rest radiates off into space. But it is enough energy to support life on Earth.