What is the relationship between half-life (radiometric dating) ad the absolute age of an object?

The radiometric dating is used to determine the absolute age of rocks. There are unstable isotope elements that decay over time into stable elements. For those whose half-life is known it can be used to calculate the age based on the ration of the amount of the unstable elements and the number of stable atoms (that are products of the radioactivity) in the rocks.