Carbon is naturally in all living organisms and is replenished in the tissues by eating other organisms or by breathing air that contains carbon.
At any particular time all living organisms have approximately the same ratio of carbon 12 to carbon 14 in their tissues.
Where t is the age of the fossil (or the date of death) and ln() is the natural logarithm function.
If the fossil has 35% of its carbon 14 still, then we can substitute values into our equation.
And then you can use that rate to actually determine how long ago that thing must've died. It would be a pretty reasonable estimate to say, well, that thing must be 5,730 years old.