Carbon Dating:
Basically when an organism is living it continuously absorbs Carbon-14 (which is a radioactive isotope of Carbon-12).
When the organism dies no more Carbon-14 is absorbed.
This means as the Carbon-14 atoms begin to decay the amount of Carbon-14 in the organism drops.
Scientists compare the amount of Carbon-14 found in the sample compared to the amount of Carbon-14 found in living things to determine the age of the sample.
Hope this helps :)