Radiometric dating is possible if a rock contains
The vast majority of carbon atoms, about 98.89%, are C12. And since carbon is an essential element in living organisms, C14 appears in all terrestrial (landbound) living organisms in the same proportions it appears in the atmosphere. Animals and fungi get C14 from the plant or animal tissue they eat for food. The C14 already in the organism doesn't stop decaying, so as time goes on there is less and less C14 left in the organism's remains.
If we measure how much C14 there currently is, we can tell how much there was when the organism died, and therefore how much has decayed.
That's the essence of radiometric dating: measure the amount that's present, calculate how much is missing, and figure out how long it would take for that quantity of the isotope to break down.
So the dates derived from C14 decay had to be revised.
One reference on radiometric dating lists an entire array of corrective factors for the change in atmospheric C14 over time.
I found several good sources, but none that seemed both complete enough to stand alone and simple enough for a nongeologist to understand them.
Thus this essay, which is my attempt at producing such a source.
Contents: The half-life of a radioactive isotope is defined as the time it takes half of a sample of the element to decay.