Radioactive dating (often called Radiometric dating) is a technique used to date materials such as rocks or carbon, based on a comparison between the observed abundance of a naturally occurring radioactive isotope and its decay products, using known decay rates.
For example say in a rock radioactive X is found and after decay say X produces Y. Say half-life of X is 1m Years. Now under examination X is found in 10 units and Y is found in 30 units. i.e. After 1m Years 40units of X converted to 20 unit of X and 20 units of Y, and after 2m years the result is 10unit of X and 30 unit of Y. So the age of rock is 2m years.