A pot still is a type of still used in distilling spirits such as whisky or brandy. Heat is applied directly to the pot containing the mash (e.g. for whisky) or wine (for brandy). This is called a batch distillation (as opposed to a continuous distillation).
At sea level, water boils at 100 degrees Celsius (212º F) but alcohol boils at 78 degrees Celsius (172º F). Therefore, in the distilling process, while there is still alcohol in the mash, the vapour is richer in alcohol than the liquid itself. When this vapour is condensed, the resulting liquid therefore contains more alcohol. In the pot still, the alcohol and water vapour, combined with vapours of the multitude of aroma components such as esters, alcohols that give the mash or wine its aroma, evaporate and flow from the still through the condensing coil. There they condense to the first distillation liquid, the so-called 'low wines', with a strength of about 25-35% alcohol by volume, which then flows into a second still below. It is then distilled a second time to produce the colourless spirit, collected at about 70% alcohol by volume. Maturation in an oak aging barrel typically causes the brown color to develop over time.