This distinction is important enough to dwell on.

The atmospheric concentration of carbon-14 has not been perfectly constant over time.

Solar activity, geomagnetic field variations, and changes in ocean circulation all affect how much carbon-14 is produced and how it is distributed through the atmosphere and biosphere.

These variations mean that the relationship between radiocarbon years and actual calendar years varies across different time periods.

Calibrating radiocarbon dates to calendar years requires comparing the radiocarbon measurements to independently dated records that span the same time period.

The primary tool for calibration going back approximately 14,000 years is the record of carbon-14 in tree rings, which provide a year-by-year record of atmospheric carbon-14 variations that can be used to convert radiocarbon years to calendar years.