In order to play cards, players must build their casting cost resources pile by sacrificing cards.

• The sacrifice phase takes place after the beginning of the turn phase and before the action phase, read more information about how a turn goes.
• Although it is possible to build resources faster than one by turn, players can only sacrifice one card per turn.
• Sacrificed cards come from hand.
• With a few exceptions, cards in the resources pile will not be seen during the game.
• Players can sacrifice a card but it is not an obligation even if strongly advised during the first rounds of play (at least until the second turn of the game).
• To help players within the digital game, the cards in their hand are highlighted with a red color outline during the sacrifice phase and the button Skip or Sacrifice are red, as shown with the two first images above.
  
  

Useful link:

• The Beginner's Guide to Sacrificing