During the maturation an amount of alcohol evaporates through the casks pores (the so-called Angel’s share). However most distilleries reduce their New Make down to 63.5% ABV before they fill it into the casks.
A Whisk(e)y has cask strength when it gets bottled without any further dilution after the maturation.
However cask strength does not necessarily mean high-proof. Depending on the age, the warehousing and the original ABV when bottled it is possible that (very) old Whisk(e)y has despite cask strength “only” an ABV between 40 and 45 percent.