The term Cask Strength means that when the whisky was removed from the cask, it was not diluted with any water, before it was bottled. Since the alcohol level of whisky is usually around 63% when it goes into the cask, it can still be high when it is selected for bottling. Most standard releases are therefore diluted with water to reduce the alcohol (most commonly to 43% or 46% in SA), as cask strength whiskies are not appealing to general drinkers.
In some cases a whisky has a higher alcohol but does not stipulate it is cask strength. Those higher strength releases are displayed here as well. (The American term for cask strength is barrel proof.)