A numerical (or "absolute") age is a specific number of years, like 150 million years ago.
Numerical dating, the focus of this exercise, takes advantage of the "clocks in rocks" - radioactive isotopes ("parents") that spontaneously decay to form new isotopes ("daughters") while releasing energy. For example, decay of the parent isotope Rb-87 (Rubidium) produces a stable daughter isotope, Sr-87 (Strontium), while releasing a beta particle (an electron from the nucleus). ("87" is the atomic mass number = protons + neutrons. Numerical ages have been added to the Geologic Time Scale since the advent of radioactive age-dating techniques. Many minerals contain radioactive isotopes. In theory, the age of any of these minerals can be determined by:
1) counting the number of daughter isotopes in the mineral, and 2) using the known decay rate to calculate the length of time required to produce that number of daughters.