Several bourbons indicate "Sour Mash" on their labels or otherwise indicate that the sour mash method was used in the creation of that bourbon. So what does that mean?
To understand what sour mash is, you first have to understand the term "mash." At the beginning of the distillation process of bourbon, cornmeal and water are mixed together at a high temperature. The temperature is then lowered, and then either rye or wheat is added depending on the bourbon. The rye or wheat are the "flavor grains," so using one or the other has an impact on the flavor profile of the final product. The temperature is once again reduced when malted barely is added. The barley is added because it releases enzymes that are necessary to convert starches into sugars, which is essential in distillation. Once all these ingredients are together, mixing a big tank at the distillery, it becomes the mash.
After sufficently mixed, the mash then goes onto the fermentation process where all the alcohol is distilled from the mash. The stuff left over after all the alcohol is distilled is known as the sour mash due to its sour flavor.
The sour mash, also called backset and spent mash among others, is then introduced to a new batch of mash when entering the fermentation process. So, sour mash is fermented mash that is left over from a previous distillation that is added to a new distillation. The process is used to control the acidity levels of subsequent fermentations, which in turn helps ensure that each distillation is consistent in flavor.
The Sour Mash method is used in the distillation of many bourbons, including Ancient Age, Evan Williams, Woodford Reserve, Knob Creek, Maker's Mark, Wild Turkey, and Old Fitzgerald. It's a very traditional way of bourbon production, still used today despite modern technology's creation of alternate methods. This adherence to tradition and heritage is one reason I enjoy American whiskey so much. Cheers.