I encountered the following sentence, the meaning of which I couldn't get even after looking up in dictionaries and googling.
Still, it might seem costless to bluff – until the hand was called. (Source)
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Those terms are originally referring to the card game poker:
One principle of the game is that the players will continue to bet money on the cards they have (called hand). If a player fears that he has the hand with the lesser value, he may fold, i.e. give up the round or bluff, i.e. continue to raise the money on the table, aka the stakes - hoping that the others will fold, under the assumption that the bluffing player’s hand is truly valuable.
At some point in the game, unless all other players fold, a player may request that the hands of the remaining players are shown, that’s the time when the value of the cards is checked and the player with the best cards wins the stake. That’s also the time when a bluff becomes obvious. If a player suspects that his opponent is bluffing, he will call his bluff by doing just this.
In your example, the poker terms are used to describe a similar pattern in politics: Making empty threats or exaggerating one’s strength or willingness to act is easy, following through may be a lot harder.
The terms are from the card game poker, which we need to understand before we can dissect your linked metaphor.
First of all, there are two related but distinct uses of the term "hand" in poker:
Within a hand, various rounds of betting occur (depending on the specific poker variation). During each round of betting, the first player can choose to either bet or check (bet $0), and subsequent players can do one of three different things:
If you have a bad hand, your only choices are to fold or to bluff (raise in the hope your opponent will fold). If you are bluffing, you don't want someone to call, because your cards will be shown and you will lose.
Let's now dissect the metaphor from your article:
Today the U.S. provides the necessary nuclear deterrent. However, why should the U.S. should remain in the middle of the Koreas’ violent quarrel?
A South Korean nuclear deterrent may not be a good outcome, but then, the only good solution is the negotiated disarmament of the DPRK, which is the least likely result. Far worse is allowing another country to hold America’s homeland hostage. But that is where present policy is leading.
Still, it might seem costless to bluff – until the hand was called. Then an administration would have to decide whether it was willing to risk the incineration of Honolulu – when missiles really were incoming – and other cities. For what? To protect a friendly nation, but not one critical to America’s security.
The subject of the article is, basically, should the United States continue to defend its ally (South Korea) against aggressive threats from North Korea, now that North Korea (reportedly) has nuclear-armed missiles capable of reaching the US?
The "bluff" is this context is military warnings/counter-threats from the US President (or other US officials) to North Korea. Are these "warnings" sincere? Would we actually follow through, if it came to it? If so, what's the danger to Hawaii or Western US states?
The "cost" of this bluff is, potentially, the nuclear war that could result if North Korea decided to "call our bluff" (by actually initiating military conflict with South Korea). At that point the US would have to "show our hand" (either by jumping in to the fight, or by backing off).