From the Comprehensive Rules 4.0:
In the upper left corner of each Free Peoples and Shadow card is that card’s twilight cost. This is the number of twilight tokens that must be added to or removed from the twilight pool to play that card.
When you play a Free Peoples card, you must add a number of twilight tokens (from the reserve) to the twilight pool equal to that card’s twilight cost.
When your opponent plays a Shadow card, he must remove a number of twilight tokens from the twilight pool equal to that card’s twilight cost. A Shadow card cannot be played if its twilight cost cannot be met by the tokens available in the twilight pool.
In game text, you will find phrases like “Add ” which means, “Add 1 twilight token to the twilight pool.”
You must meet any requirements to play a card (or perform an action) before paying its costs.
. . .
All Shadow players pay for cards by using the same twilight pool. The second Shadow player uses twilight tokens left over from the first Shadow player, and so on.
Cards already in play that affect another card’s twilight cost have that effect only when the card to be modified is coming into play.
See also cost, discard pile, draw deck.