The term generally means that if the cards are face down in a deck, you can not discern one card from another. Rips, folds, and obvious edge damage or discoloration would make cards discernable.
As an extreme example, if you have one notch on the top right to signify Son of God and two notches on the middle left to signify New Jerusalem, you could conceivably know what cards are coming in future draws which would impact your decision-making (a.k.a. cheating).
