Each atom of an element has a fixed number of protons and electrons, however, atoms can have different amounts of neutrons. Isotopes are atoms of an element that have different numbers of neutrons. Some isotopes are unstable and can emit radiation.
An atom of carbon most commonly has 6 protons, 6 electrons, and 6 neutrons. Each proton and neutron has a mass of 1amu. If you add the mass of 6 protons and 6 neutrons you can identify that carbon usually has a mass of 12amu.
When carbon has a mass of 14amu it has more neutrons than usual. Since carbon atoms always have 6 protons, the carbon 14 isotope of carbon has 8 neutrons.
Calculating atomic mass:
To calculate the average atomic mass of an element, average the percent abundance of the existing isotopes.
0.1% Carbon -14
To find the average atomic mass of carbon, multiply the percent as a decimal of each isotope by the mass of that isotope and add them together.
(0.989)(12amu)+(0.0111)(13.0039amu)+(0.001)(14amu) = Average mass of carbon = 12.026amu