In casino poker, players are often allowed to "muck" their cards. This means that, after a showdown, they are allowed to concede the pot without revealing their cards to the other players. Although technically players are allowed to view the mucked cards, humans will usually not look as a matter of courtesy.
If there is no mucking, then all players will reveal their cards at every showdown. But, if a player folds before the showdown, then their cards will not be revealed until after the match is over.