Clarified butter is just the oily part of butter, i.e. without the buttermilk. It gives potatoes a rich, buttery flavour. It will heat to a higher temperature than ordinary butter without burning. You can buy it ready prepared as ghee.
However, it’s easy to make: To make about 200ml, take 250g of unsalted butter and melt on a low heat in a small saucepan. Allow it to stand for a few minutes until all the oil rises to the top, then skim off the oil into a sealable plastic container. It will keep for 2 months in the fridge.