Radiocarbon is a naturally occurring radioactive isotope of carbon. Atoms of radiocarbon behave in the same way as any other carbon atoms except that they weigh slightly more and, after an average of some 5000 years decay to nitrogen.
Because radiocarbon is a radioactive isotope, it only exists on the earth because it has been formed recently. We know from nuclear physics that several possible reactions can result in radiocarbon. These all involve neutrons.
The details are not important in understanding radiocarbon but the possible reactions are: