There are two techniques for dating in archaeological sites: relative and absolute dating.Relative dating stems from the idea that something is younger or older relative to something else.The extra neutrons in Carbon-14’s case make it radioactive (thus the term, radiocarbon).
More recently is the radiocarbon date of 1950 AD or before present, BP.
How It Works: Carbon has 3 isotopic forms: Carbon-12, Carbon-13, and Carbon-14.
The numbers refer to the atomic weight, so Carbon-12 has 6 protons and 6 neutrons, Carbon-13 has 6 protons and 7 neutrons, and Carbon-14 has 6 protons and 8 neutrons.
In fact, levels of Carbon-14 have varied in the atmosphere through time.
One good example would be the elevated levels of Carbon-14 in our atmosphere since WWII as a result of atomic bombs testing.