What is radioactivity?

Imagine a water ballon which is slowly filling up with water. The ballon is fine and intact when there's not much water in it but what happens when we fill it so much that it can no long hold all the water? It burst! Now we can think of radioactivity in this way. The nucleus of an atom has protons and neutrons in it (the water of the balloon) and they are all held together by a force (the balloon itself). If we over fill the nucleus it can evetually burst and we call this decay. During decay either a particle or a wave may be emitted which we refer to as radiation and heat is also given out. The process is entirely random so we can never predict when an unstable nucleus will decay.