Complex sentence: Single malt whisky is malt whisky from a single distillery, that is, whisky distilled from fermented mash made exclusively with malted grain (usually barley), as distinguished from unmalted grain.
Split into simpler sentences: Single malt whisky is malt whisky from a single distillery. Malt whisky is whisky distilled from fermented mash made exclusively with malted grain (usually barley), as distinguished from unmalted grain.