Criticality is a nuclear energy term and it occurs when a nuclear chain reaction in a mass of fissile material is self-sustaining, in other words the mass produces energy by itself without any external influence. In nuclear experimentation criticality can get out of control and produce what is known as a criticality accident, which is an accidental increase of nuclear chain reactions in a fissile material, such as enriched uranium or plutonium. This releases a surge of neutron radiation which is highly dangerous to humans and causes induced radioactivity in the surroundings, and can be extremely deadly.
Below is a clip from the movie Fatman and Little Boy (1989), a Hollywood recreation of the years of the development of the bombs that devastated Hiroshima and Nagasaki during WWII. In this scene nuclear scientist (John Cusack) during and experiment accidentally lets 2 cores of plutonium reach criticality releasing a huge amount of radiation that eventually ends up killing him. This is one of the most famous criticality accidents in history.
Fictionalised criticality accident in the movie Fatman and Little Boy