Unstable heavy nuclei decay into lighter particles by emitting nuclear radiation. It doesn't always happen that the product of the decay, called the daughter nucleus, is stable. If the daughter nucleus is unstable, then eventually it will decay into a lighter particle. The result may be that a series of decays occurs, until after several decays, a stable nucleus results.
One possible decay series is shown below. This is only an example of a decay series.
This decay series is illustrated below.
Each decay results in the release of energy, as radiation/kinetic energy of the emitted particle and the daughter nucleus.