While not all objects have the same isotopes, both living and nonliving objects have some sort of decaying, radioactive isotope that can be used based on known decay rates. An isotope of some sort is located and isolated within an object.
That isotope is then compared to its decaying product and scientists are able to use known decay rates to determine how old the initial isotope is.
I've googled it but I really don't understand it.
This works because elements have a life cycle known as a “half-life.” A half-life refers to the amount of time it takes for an isotope to lose half of its atoms as a result of decaying.
When an isotope decays, it often becomes a different kind of element altogether.
Radiometric dating (often called radioactive dating) is a technique used to date materials, based on a comparison between the observed abundance of particular naturally occurring radioactive isotopes and their known decay rates.
It is the principal source of information about the absolute age of rocks and other geological features, including the age of the Earth itself.