Calibration of radiocarbon determinations is in principle very simple. If you have a radiocarbon measurement on a sample, you can try to find a tree ring with the same proportion of radiocarbon. Since the calendar age of the tree rings is known, this then tells you the age of your sample.
Why do carbon-14 ages need to be calibrated?
Calibration of radiocarbon results is needed to account for changes in the atmospheric concentration of carbon-14 over time. These changes were brought about by several factors including, but not limited to, fluctuations in the earths geomagnetic moment, fossil fuel burning, and nuclear testing.
What happens to carbon-14 but not carbon 12 in a living organism after it has died?
When an organism dies, it ceases to absorb Carbon 14 from the atmosphere and the Carbon 14 within the organism decays exponentially, becoming Nitrogen 14, with a half-life of approximately 5730 years. Carbon 12, however, is stable and so does not decay over time.