All radioactive isotopes have a characteristic half-life (the amount of time that it takes for one half of the original number of atoms of that isotope to decay).
For an example of how geologists use radiometric dating, read on: A geologist can pick up a rock from a mountainside somewhere, and bring it back to the lab, and separate out the individual minerals that compose the rock.
They can then look at a single mineral, and using an instrument called a mass spectrometer, they can measure the amount of parent and the amount of daughter in that mineral.
This is an enormous branch of geochemistry called Geochronology.
It is an accurate way to date specific geologic events.
For example Carbon 14 has a half life of 5,730 years.
If a sample has 6.25 % of the amount of Carbon 14 present in a living sample then four half lives have passed since the sample died and stopped absorbing Carbon. Carbon 14 has a limited of approximately 50,000 years or less.Divide the percentage in half again 12.5% left three half lives have passed.Again divide it in half 6.25% and four half lives have passed.The reason that I trust the accuracy of the age that we have determined for the earth (~4.56 billion years) is that we have been able to obtain a very similar result using many different isotopic systems.Most estimates of the age of the earth come from dating meteorites that have fallen to Earth (because we think that they formed in our solar nebula very close to the time that the earth formed).For example, the element Uranium exists as one of several isotopes, some of which are unstable.