What is the meaning of Potash?

The water-soluble part of wood ash, used for making soap and glass and as a fertilizer (chemically speaking, an impure form of potassium carbonate (K2CO3) mixed with other potassium salts).

Potassium. Chiefly used in the names of compounds of the form "... of potash".

A class of potassium minerals, of similar applicability to potassium carbonate, such as being a fertilizer. (ie. potassium chloride, potassium hydroxide)

To treat with potassium.

Source: wiktionary.org