Living organisms are constantly incorporating this C-14 into their bodies along with other carbon isotopes.
When the organisms die, they stop incorporating new C-14, and the old C-14 starts to decay back into N-14 by emitting beta particles.
The older an organism's remains are, the less beta radiation it emits because its C-14 is steadily dwindling at a predictable rate.
So, if we measure the rate of beta decay in an organic sample, we can calculate how old the sample is. Question: Kieth and Anderson radiocarbon-dated the shell of a living freshwater mussel and obtained an age of over two thousand years.
ICR creationists claim that this discredits C-14 dating. Answer: It does discredit the C-14 dating of freshwater mussels, but that's about all.