FRATIONAL ABUNDANCE
Atomic weight are average atomic masses, the sum of the mass of each naturally occurring isotope of an element times its fractional abundance.
OR
All elements exist as isotopes in nature. This means that carbon has 6 protons, but some varieties have 6 neutrons and some may have 8. This would be denoted as carbon - 12 and carbon -14
A fractional abundance simply means, what is the mixture of these isotopes in nature. or the carbon example above, approx. 99% is the carbon -12 variety and less than 1 % is carbon-14.
When they are averaged together, based on this relative presence in nature, you get an average atomic mass of 12.011 .
NATURAL ABUNDANCE
In chemistry, natural abundance (NA) refers to the abundance of isotopes of a chemical element as naturally found on a planet. The relative atomic mass (a weighted average) of these isotopes is the atomic weight listed for the element in the periodic table. The abundance of an isotope varies from planet to planet, and even from place to place on the Earth, but remains relatively constant in time.
As an example, uranium has three naturally occurring isotopes: 238U, 235U and 234U. Their respective NA range from 99.2739 - 99.2752%, 0.7198 - 0.7202%, and 0.0050 - 0.0059%.[1] For example, if 100,000 uranium atoms were analyzed, one would expect to find approximately 99,275 238U atoms, 720 235U atoms, and no more than 5 or 6 234U atoms. This is because 238U is much more stable than 235U or 234U, as the half-life of each isotope reveals: 4.468×109 years for 238U compared to 7.038×108 years for 235U and 245,500 years for 234U. However, the natural abundance of a given isotope is also affected by the probability of its creation in nucleosynthesis (as in the case of samarium; radioactive 147Sm and 148Sm are much more abundant than stable 144Sm) and by production of a given isotope by natural radioactive isotopes (as in the case of radiogenic isotopes of lead).