A lye is a metal hydroxide traditionally obtained by leaching ashes (containing largely potassium carbonate or "potash"), or a strong alkali which is highly soluble in water producing caustic basic solutions. In our case specifically we use a sodium hydroxide.

1. Sodium hydroxide is also known as caustic soda.
2. Caustic means "burning".
3. It has the chemical formula of NaOH.
4. It is a base, meaning it has a very high pH.
5. It is the most common thing used to raise the pH of things; for example, to neutralize an acid.
6. It dissolves easily in water, and makes the water warm when this happens. This means that it is an alkali.
7. Sodium hydroxide is used as a solution named Lye to make soap.
8. Its dissolving properties and its ability to easily break surface tension are fundamental for its uses.

Sodium hydroxide can be made using the chlor alkali process. in the simplest terms watch this video or read below for the TLDR version.

To make lye in the kitchen, boil the ashes from a hardwood fire (soft woods are too resinous to mix with fat) in a little soft water, rain water is best, for about half an hour. Allow the ashes to settle to the bottom of the pan and then skim the liquid lye off the top.