What is radioactive decay and how is it used to determine the absolute age of a rock?

Radioactive decay is when an unstable isotope releases subatomic particles in order to become more stable. Unstable isotopes decay at a known rate called a half life. A half life is the amount of time it takes half of the isotope to decay. Also, the unstable isotope is called the parent isotope, while the one it decays into is called the daughter isotope. By measuring the amounts of parent isotope in comparison to the amount of daughter isotope in a sample, one can get an idea of how old the sample is.