Potash is a wood-based product which has applications in farming, as well as production of mid- and high-end glass products.
 Producing potash
Potash can be created at an ashery by a dwarf with the potash maker labor enabled. This process requires one unit of either ash or lye, though as lye is produced from ash, going that route is less efficient with no added benefit. As ash must be produced by burning logs at a wood furnace, production of one unit of potash requires one unit of wood and two (ash route) or three (lye route) actions.
Potash is produced in bars with no quality level. These bars can be stored in bar stockpiles, and are viewable in the stocks screen under "bars."
Potash can be used to fertilize a farm plot, increasing the stack size of harvested plants. Doing so can help cut down on the need for large farms, which may result in less labor overall.
Alternatively, potash can be further processed into pearlash at a kiln or magma kiln. This material is necessary if you wish to produce glass products of clear or crystal quality.
Finally, potash is a magma-safe (though low-value and eccentric) building material, and can be used in any constructions.
 Real world
Potash refers to a wide variety of potassium bearing salts, first produced in roughly 500 CE. In modern times, they are used primarily in production of fertilizer, though they were once part of glass and soap production. While modern production methods have developed, this substance was once produced by soaking plant ash in a pot full of water, thus the name pot-ash.
[MATERIAL:POTASH] - reconstructed from data extracted from memory