A single cask whisky, means that the release is the result of only one cask and thus will be limited in the number of bottles produced. Most standard whiskies are not from a single cask, but from a vatting of multiple casks (even single malts) in order to produce enough bottles. Single casks however, are limited by nature since it is restricted to the size of the cask that is being bottled. Casks do come in different sizes, but by law, for Scotch whisky a cask cannot be larger than 700L, and is more often a 250L hogshead or a 500L butt.
Whisky Tip: The American terminology for a cask is a barrel, but in the whisky industry a barrel is a specific type and size, whereas a cask is the umbrella term covering all shapes and sizes.