Original: The most dangerous cyanides are hydrogen cyanide (HCN) and salts derived from it, such as potassium cyanide (KCN) and sodium cyanide (NaCN), among others.
Simplified: The poisons referred to are usually hydrogen cyanide (HCN), and the chemicals which are similar to it, like potassium cyanide (KCN), and sodium cyanide (NaCN).