If you are asking about the fuel the powers the Sun in its core - that is hydrogen combining to form helium and energy in a process called nuclear fusion.
In nuclear fusion mass is converted into energy.
That is what Einstein's famous equation E=mc^2 describes.
"m" is the mass being converted into energy, "E," and "c^2" is the speed of light times itself, which is a very large number:
c^2= c * c = 299,792 km/s * 299,792 km/s = 90 billion (km/s)^2
That is a big number so just a little bit of mass gives you a lot of energy!

Terry