Simple definition of carbon dating
Carbon dioxide in the atmosphere contains a constant amount of carbon-14, and as long as an organism is living, the amount of carbon-14 inside it is the same as the atmosphere.
However, once the organism dies, the amount of carbon-14 steadily decreases.
In the late 1940s, American chemist Willard Libby developed a method for determining when the death of an organism had occurred.
He first noted that the cells of all living things contain atoms taken in from the organism's environment, including carbon; all organic compounds contain carbon.
You will notice that after around 40,000 years (or 8 half-lives), the amount left is starting to become very small, less than 1%.
It is often used on valuable artwork to confirm authenticity.
Carbon 14 has a half-life of 5,780 years, and is continuously created in Earth's atmosphere through the interaction of nitrogen and gamma rays from outer space.
Because atmospheric carbon 14 arises at about the same rate that the atom decays, Earth's levels of carbon 14 have remained fairly constant.
Once they die, they stop taking in carbon-14, and the amount present starts to decrease at a constant half-life rate.
Then the radiocarbon dating measures remaining radioactivity.