Radiocarbon age, calibration curve কী?

The (conventional) radiocarbon age of a sample is

where $A_{\mathrm{SN}}$ is the activity of the sample’s carbon and $A_{\mathrm{ON}}$ that of the reference standard; it uses the half-life of $5568$ years (mean life $8033$ years), normalises the activities for the isotopic fractionation of carbon ($\delta^{13}\mathrm{C} = -25\,\text{‰}$), and counts years before AD 1950 (BP, “before present”). A calibration curve gives the radiocarbon age of samples of known calendar age; read backwards, it converts a radiocarbon age into a range of calendar ages (cal BP).