The percentage indicates the proportion of alcohol in the whisky. For example:

When you pick up a bottle of whisky, one of the most prominent numbers on the label—aside from the age statement—is the percentage. Whether it’s 40%, 46%, or a cask-strength beast pushing 60%, this number dictates everything about your drinking experience, from the aroma hitting your nose to the burn sliding down your throat.

Historical Note: The term "proof" originated in the 16th century. British sailors would test their rum rations by soaking a gunpellet in the spirit. If the gunpowder could still ignite, the alcohol content was "proven" to be high enough (above 57% ABV). If it didn't, the rum was considered "under proof" and potentially watered down.