Atoms are made up of three key ingredients: protons, neutrons and electrons. The nucleus is the center of the atom, made of just protons and neutrons, and electrons orbit the nucleus.
A nucleus with a specific number of neutrons and protons is called a nuclide. Nuclides that are made up of a balanced mix of protons and neutrons are stable, meaning they are very hard to break down. Adding or removing neutrons and protons to a stable nuclide can make it less stable.
When an unstable nuclide breaks apart it emits radiation. We call this process radioactive decay.