Carbon dating is one of the uses of radio active isotopes. The technique will help to find out the age of archaeological objects (wood, plant, and animal fossils). the principle is based on the fact that all living matters contain a definite amount of radioactive isotope carbon 14.
Carbon 14is formed in the upper atmosphere by the bombardment of N-14 by cosmic rays. Some of it is present in the carbon dioxide. In the photosynthesis process, plant absorb some C-14. As the animals live on plants they acquire some C-14. A plant or any living being bduirng its life time maintains a reasonable balance of C 14 in its tissues. From the death of the plant, it will not absorb any more c-14. But C-14 starts decaying.
Therefore by having an estimate of C-14 that will be normally there in a tissue and by determining the amount of C-14 now in the dead tissue, the age of the dead tissue can be determined.