Smoking, or fuming wood, as it is often known, is a technique used to darken and change the colour of woods which are rich in Tannins, such as Oak, Chestnut, Cherry, or Mahogany.
The process of smoking / fuming involves exposing the wood to ammonia gas in a sealed chamber. The tannins in the wood react with the ammonia causing a chemical reaction which results in the wood darkening and changing colour. The effect can be really quite dramatic turning a light wood dark very quickly. Traditionally Oak used to be put into horse manure (rich in ammonia) to create a similar effect!
Once a wood which is rich in tannin has been smoked / fumed, it will retain the new colour.