Cask Strenght means high alcohol. Whisky is typically diluted with water to about 40% ABV. However, cask strength stuff isn’t messed with, so it can remain anywhere between 50-75% ABV. As a rule of thumb, expect a cask strength dram to be around 60-65% ABV.
Barrel strength and overproof are two more expressions to denote the liquid’s strength. Historically, alcohol was ‘proofed’ by mixing it with gunpowder. If the alcohol content were over 50%, the gunpowder would ignite and seamen would happily accept their overproof Rum. ABV is usually half the overproof figure; thus100 proof is 50% ABV.