Peat is a brown, soil-like material characteristic of boggy, acid ground, consisting of partly decomposed vegetable matter. It is widely cut and dried for use in gardening and as fuel.
Many old distilleries burn peat to dry the barley grain to give it their own level of smokiness. Drying over peat generally lasts 18 to 30 hours. Other distilleries prefer unpeated fires to avoid the smoky flavor in their whisky.
Here’s a brief video explaining the peat gathering process from the Isle of Islay.