Atoms may have a stable or an unstable nucleus. Atoms with an unstable nucleus can decay spontaneously without any external influence. During decay, ionising radiation is produced. This is known as (natural) radioactivity.
Decay may also be induced artificially in a process known as nuclear fission. In a nuclear reactor, a controlled chain reaction of nuclear fission events produces heat at a steady rate. Ionising radiation is emitted as well.
There are various types of ionising radiation. They can cause damage to human tissue, so radiation protection is extremely important.