All cards except any retained by generals from the previous turn. You generally want to retain unplayed YELLOW or RED cards. You may want to retain unplayed GREEN cards. Normally you would want to discard any unplayed WHITE cards and you would invariably want to discard unplayed BLACK cards.
All the cards used to prompt actions, used as discards or unplayed cards are returned to the pack. The pack is shuffled and the cards then dealt out to bring the generals back up to their card allowance.
Richard