The second stage of whisky-making is basically a brewing process. In brief, the malt is brewed to turn the sugar into alcohol.
The grist is mixed with hot water in the mash tun to dissolve the sugar. This sugary liquid (wort) is the piped into wooden vats called wash backs. The wort in the wash backs is mixed with yeast to turn the sugar into alcohol. This produces a weak beer (wash) which is then taken to the stills.