I think that the key is lye, but someone else should be able to say that more authoritatively.
Lye more correctly sodium hydroxide or potassium hydroxide is used in soap making. You cannot make soap without it. Basically soap is a salt made by combining an acid (the fatty acids found in fats or oils) with a base (NAOH or KOH). That is why if you look on a bar of soap you will see ingredients such as sodium palmate, sodium cocoate, and sodium olivate. This tells you that palm, coconut,and olive oil have been saponified by the use of sodium hydroxide.