To reveal (or show) a card is to show the card's face to all players.
Cards in hidden zones (such as the hand or Deck) may not be voluntarily revealed; they can only be revealed when a card effect or cost instructs a player to do so, or to verify to the opponent that the card is what the player says it is.
After a card is revealed, it normally returns to its original position. Revealing the card does not activate it, nor does it activate or apply its effect. Revealing a face-down card does not count as flipping it.
If a card is already public knowledge, it cannot be revealed. For example, if "Mind on Air" is face up on a player's side of the field (with its effect active), their opponent cannot activate the effect of "Golden Ladybug".
Reveal for verification
If an effect causes a player to search their Deck for a card with specific qualities and then add it to their hand (or Set it to their field, place it on top of the Deck or otherwise move it to a hidden position or zone), that player must reveal the card to their opponent after taking it out from their Deck and before placing it in its destination. For example, when a player adds a Monster Card to their hand using the effect of "Sangan", their opponent may read all of the information on the Monster Card before it is added to the player's hand.
In the case where an effect requires a hand to be revealed, all players may examine every card in the hand. For example, if a player uses "Trap Dustshoot", they may look at all the cards in their opponent's hand, even the non-Monster Cards.