Well, when an organism (Example: human) dies it  approximately has 10g of carbon-14 present in it, as it begins to decay the carbon-14 being radioactive starts to decay too .Depending on the amount of carbon-14 found in the body we can determine the age of the specimen found.For example the half life of carbon-14 is 5730 years,which means it takes 5730 years for half the carbon-14 in your body to decompose, suppose you find 5g of carbon-14 in the human body you find you can say the human lived 5730 years ago.The amount of carbon-14 in a body is measured using a Geiger Muller counter.