KH is a term used for 'carbonate hardness', the 'K' comes from the German 'karbonate', and KH is also referred to as 'German Hardness'.
Water hardness is a measure of the amount of dissolved minerals in the water, the more there are, the harder the water is.
Carbonate hardness is important to 'buffer' the aquarium water by neutralising acidic compounds and preventing fluctuations in pH.
Hard-water areas contain water with plenty of carbonate hardness although over time this may diminish in the aquarium.
For fish keepers who use pure water or very soft water, mineral additives may have to be added to maintain a suitable KH level.