A radioactive isotope is an atom that is unstable, and the nucleus will fall apart or "decay."  Carbon-14 is a radioactive isotope of carbon, it occurs naturally, and you have millions of radioactive carbon-14 isotopes in your body.

When one of these carbon-14 atoms decays, some of the mass of the nucleus (protons & neutrons) is converted into energy and leaves the nucleus as radiation.  The residual protons and neutrons grab some electrons and reassemble into some other atom.