What does an unstable isotope give off that a stable isotope does not?

An unstable isotope emits some kind of radiation, that is it is radioactive. A stable isotope is one that does not emit radiation,

Many elements have one or more isotopes that are radioactive. These isotopes are called radioisotopes. Their nuclei are unstable, so they break down, or decay, and emit radiation. The nucleus may be unstable because it has too many protons or an unstable ratio of protons to neutrons.