What this means, i read about that Three trials thing and there was this card. It could be nice that somebody would tell me what this secrecy thing do?

Secrecy gives you a discount on the cost of the card when your threat is 20 or lower.

So for example, if a card costs 4 and has secrecy 3 on it, then if your threat is below 20 you only need to pay 1 resource.

If above 20, you need to pay the full price of 4.

I guess that "secrecy" keywords would stack with any existing "secrecy x" on the played card then ?

(as archery keywords stack on existing archery, like with Glade of Cleansing )

What if I have several "leaf brooch" on different heroes from the same sphere ?

They stack, it says so at the end of the article somewhere.

So in theory, a mono sphere deck with 3 brooches attached to its heroes would be able to cast event cost 3 cards for free, as long as your threat is below 20.

I WANT THAT CARD!