IIRC, Masked was a keyword in GT that let you play an attack facedown. You told your opponent if it was Physical or Energy, and he/she could choose to block the attack. Once they decided to defend or not, you'd flip the card over, pay for the attack, and resolve any effects of the card that were not stopped or prevented. If you played a Masked attack that you couldn't pay for when you played it, you lost the game.
I don't remember what would happen if the opponent's block did something to prevent you paying (e.g. You Mask an energy attack that costs 2 PS, but your opponent's block makes you lose 10 stages.)