It is functionally short for "While this card is in your hand or in your Champion Zone on your turn during an Open State, you may pay [A] to hide this facedown at a battlefield you control that doesn't already have a facedown card hidden there for as long as you control that battlefield. Beginning on the next turn, this gains [Reaction] and you may play this, ignoring its base cost."
§ 811.1.b Core Rules PDF (officieel)
§ 811 Hidden
§ 811.1 Hidden is a keyword that acts as a prerequisite to perform the Hide Discretionary Action.
