Reduction is when something is made (smaller / or shrinks) or is reduced, decreasing the size, price or quantity.
Reduction (Deoxidation) takes place in the firing of ceramics with gas or wood. The oxygen in the kiln is reduces and / or eliminated, combustion js create and the smoke that is produced, that contains particles of carbon or carbon monoxide. These particles lodge in the pieces (except where there are reservations) and alter the colors.
The exception is the Raku pottery, where the reduction takes place outside the kiln while the piece is still is red hot.