The complication is that the ratio of carbon-14 in the atmosphere has not been constant over time.

It varies in response to changes in cosmic ray intensity, solar activity, and other factors.

If you simply assume that the atmospheric carbon-14 ratio was always the same as today, the raw radiocarbon dates will be systematically off.

Calibration corrects for these variations using records of atmospheric carbon-14 concentrations derived from tree rings, coral, cave formations, and other independent archives.

For samples from the late Pleistocene, calibration curves can shift apparent ages by several hundred years.