Some elements, and many isotopes, have what is called an "unstable nucleus". This means the nucleus is either too big to hold itself together or has too many protons or neutrons. When this happens the nucleus has to get rid of the excess mass or particles. It does this through radiation. An atom that does this can be called "radioactive". Unstable atoms continue to be radioactive until they lose enough mass/particles that they become stable. All atoms above atomic number 82 (82 protons, lead) are radioactive.