It's about the natural chemical Amygdalin (or Nitriloside) found in foods like apricot seeds and bitter almonds, that is virtually harmless to healthy cells in moderate amounts, but are lethal to cancer cells.
According to the documentary, in parts of the worlds where the local diet is naturally high in Nitrilosides, cancer is almost non-existent!
The supplement of Amygdalin was marketed under the name of Laetrile, or "Vitamin B17."