What is a sutler?

The simple definition of a sutler is, A peddler who follows an army to sell goods and food to the soldiers. Wikipedia elaborates:

... These merchants often followed the armies of the American Revolution and the American Civil War to try to sell their merchandise to the soldiers. Generally, the sutlers built their stores within the limits of an army post or just off the defense line, and first needed to receive a license from the Commander prior to construction; they were, by extension, also subject to his regulations.

Sutlers, frequently the only local suppliers of non-military goods, often developed monopolies on simple commodities like tobacco, coffee, or sugar and rose to powerful stature. Since government-issued coinage was scarce during the Civil War, sutlers often conducted transactions using a particular type of Civil War token known as a sutler token.