When an organism, eg a tree, dies it stops taking in carbon dioxide.
The amount of carbon-14 in the wood decreases with time as it decays into nitrogen with a half-life of about 5700 years.
The 235U–207Pb cascade has a half-life of 704 million years and the 238U–206Pb cascade is considerably slower, with a half-life of 4.47 billion years.
So when a mineral grain forms (specifically, when it first cools below its trapping temperature), it effectively sets the uranium-lead "clock" to zero.
The age of a rock can be calculated if its ratio of uranium to lead is known.
This is unstable and eventually decays to lead, which is stable.
Uranium-Lead dating is a radiometric dating method that uses the decay chain of uranium and lead to find the age of a rock.