What does this have to do with the prodigious energy requirements of the Sun? If one
adds up the mass on the left hand side of the equation 
, one finds
that there is about 0.7 % mass than the right hand side of the equation. We have:
Mass of 4 hydrogen atoms: 
kilograms (atoms are little things)
Mass of 1 helium atom: 
kilograms
difference: 
kilograms.
It is small but measurable, and definitely there.
So what? Einstein's famous relation between mass and energy says mass can be changed into energy and vice-versa. The relation relating the two is
where c is the speed of light.
Nuclear fusion reactions can be incredible sources of energy. Example: take
1 kilogram of hydrogen. If we burned it, we'd get about 
Joules of energy. How
much from fusion reactions? If 1 kilogram of hydrogen 
proton- proton
cycle, you get 993 grams = 0.993 kilograms of helium. 0.007 kilograms has been converted
into energy. Therefore
Joules!
Summary in words. Link to the Manhattan Project.