From the FFG article here:

“I think no good of our course from beginning to end, as you know well, Gandalf,” answered Aragorn. “And perils known and unknown will grow as we go on. But we must go on; and it is no good our delaying the passage of the mountains. Further south there are no passes, till one comes to the Gap of Rohan. I do not trust that way since your news of Saruman. Who knows which side now the marshals of the Horse-lords serve?”
    “Who knows indeed!” said Gandalf. “But there is another way, and not by the pass of Caradhras: the dark and secret way.”

    –J.R.R. Tolkien, The Fellowship of the Ring

The heroes of Middle-earth undertake great and perilous quests in The Lord of the Rings: The Card Game, and their threats increase the more fully they come to the Dark Lord’s attention. As in Tolkien’s epic fantasy, the heroes find themselves in the role of the underdogs, a small band setting forth into dangerous wilds and treacherous mines, filled with Orcs, Trolls, and other monstrous enemies.
Among the first choices players must make are which heroes to include in their fellowships. Last week, Nate French explained some of the reasons the rules allow each player to select up to three heroes, and as a rule, it has historically been foolish to choose any fewer. However, this week we explore the development of Secrecy and the impact it may have upon your starting fellowship.
Smart and secret
When we announced The Redhorn Gate back in October, we hinted at the Secrecy mechanic the Dwarrowdelf cycle develops. In January’s article,Boromir’s Guide to Deck Building, Part Two, we defined Secrecy in its simplest terms and we discussed how it can give players a means to accelerate their resources. Players who can keep their threat at 20 or below gain greater card effects at discounted prices.
Still, Secrecy does something more than just accelerate resources. It opens up an entirely different path for deck construction. Because the safest way to ensure you can trigger the discounts of your Secrecy cards is to start with 20 or less threat, Secrecy encourages the design of fellowships with only two heroes (or even just one!).
Today, Lukas Litzsinger, the lead developer of the Dwarrowdelf cycle, discusses how the Secrecy mechanic took shape:
The evolution of Secrecy
While the Secrecy cards introduced in the Dwarrowdelf cycle can be used in any deck, they are easily most useful when you keep your threat at 20 or below. Players can use cards such as The Galadhrim’s Greeting (Core Set, 46) orGandalf (Core Set, 73) to lower their threat, but in order to take advantage of Secrecy from the very beginning of the game, you need a low starting threat, which may lead players to experiment with decks that play two heroes, or even just one.
However, Secrecy didn’t start in its current format. In fact, the mechanic went through a lot of revisions. Because the game already provides lots of incentives for players to lower their threat, anything that rewarded players for lowering their threat as much as possible had to be carefully implemented. Originally, we developed Secrecy cards that could only be played by players who ran just one hero. That was too restrictive, so we tried with two. Fine-tuning the cards became a major concern. If the Secrecy cards were too good, then players who started with three heroes would sacrifice their heroes just to play them, or they might be even more devious and play a bunch of Secrecy cards before reviving their hero with a Fortune or Fate (Core Set, 54). If the Secrecy cards weren’t extremely powerful, then no one would bother playing without a third hero just to use them. Ultimately, we deemed that restricting the mechanic based on the number of starting heroes (or even the number of characters in play) was too problematic.
Basing their play on a threat restriction felt more thematic. Threat already represents how visible your party is to enemies, so making Secrecy effects playable only by players with extremely low threat worked thematically. Since your heroes also set your starting threat level, running heroes with very low threat, or running fewer than three heroes makes it easier to play Secrecy cards without having to draw a lot of threat reduction. It also makes it easier to keep your threat low while the encounter deck tries to raise it.
The last change the Secrecy mechanic saw was the introduction of the variable cost reduction. At first, Secrecy functioned like an on/off switch, but players were too heavily penalized when their threat skyrocketed, making them completely unable to play a bunch of cards in their decks. So the Secrecy discount allows for a middle-ground, requiring a set threat level, but then giving various discounts to the cards. You can still play Secrecy cards without Secrecy, and building a deck with Secrecy cards that suffers an unforeseen spike in its threat level does not leave you with a bunch of unplayable cards in your hand. 
Secrecy will open up new strategies for players, and make them reconsider the default rule of three heroes to a deck.