Peat is decomposed organic plant matter that has been compressed in the ground for thousands of years. It is essentially young coal.
The “peaty” flavour in Scotch occurs during the malting process when peat is burned in the kiln. When it is burned, peat produces an especially aromatic smoke which imbues the drying barley with compounds called ‘phenols’ and gives it the typical flavours of smoke, tar, ash and iodine.
Peaty’ness is measured in phenol parts per million. A lightly peated whisky may measure up to 10-20 parts, while there will be a very noticeable peated note to whiskies above 50ppm. The full-blown bonfires at the bottom of your glass and TCP at the back of your throat weigh in at over 150ppm.