Suppose we have a tank partly filled with water, and a hole in the bottom through which the water is leaking out of the tank.We wonder how long the hole has been there, that is, how old the hole is.
As another example, a date that is obviously wrong would confirm a strong belief in the fundamental unreliability of radioactive dating.
Usually, different amounts of lead will have been lost, for example more from the surface of the crystal than from the interior, so there will be a range of isotope ratios.
If the differences are due to one episode of loss of lead, then these points will lie on a straight line, as is often observed.
That result is compared to decay curves to get a time interval.
That time interval is compared to calibration information and corrections made for known variations.