You may be familiar with the Einstein relation
$$ E=mc^2$$
Free protons and free neutrons have particular masses, which you can measure by seeing e.g. how they accelerate in response to electromagnetic forces. You can also measure the masses of other nuclei. What you find is that stable nuclei are always less massive than their protons and neutrons would be if you measured them separately. Furthermore, if you measure the energy released when two nuclei fuse into a heavier nucleus, the total energy released is equal to the mass difference between the starting components — with the unit conversion factor $c^2$ from the Einstein equation.
The mass differences are small, typically less than 0.1% of the nucleon masses. If you are doing chemistry, it’s fine say that “the proton and neutron each have mass 1” and “helium has mass 4.”
But a helium nucleus is less massive than two protons and two neutrons. That difference, or quantities related to it, are variously called the “mass deficit,” flipped around into a related quantity called the “mass excess,” or converted to energy units and called the “binding energy.”
In the sun, the “proton-proton process” turns four protons into a helium nucleus. (Two protons change “flavor” to neutrons in the process.) The “binding energy,” released as the protons are bound into the helium nucleus, explains why the sun is hot.
Your plot shows these mass differences. But it’s not helpful to directly compare the mass of hydrogen, $A≈1$, directly to the mass of iron, $A≈50$-ish. This graph tells you that, if you started with fifty-ish hydrogens and fused them to iron, you would release some of their mass as energy. But if you tried to fuse two irons into tin, $A≈100$-ish, that would cost energy. The energy difference gets big enough for very heavy nuclei that they sometimes fission on their own. A big pile of wet uranium will spontaneously release a bunch of mid-mass elements like cesium, barium, iodine, and xenon, as well as a substantial amount of heat.
