It allows players to pick a card to be exiled and marked to be cast later. When a card with Hideaway’s condition gets fulfilled, the card that was Hidden goes to the player’s hand with full mana.
This is literally one of the first cards I put in all my green EDH decks. In green whether I go tokens or Voltron or Naya Artifacts, I always have 10 power and there’s always something worth more than 2 mana to hide. Here, twenty power is the perfect translation