To pay a cost is an action required in order to activate a card, an
effect, or to Summon a monster. Tributing to Tribute Summon a
Level 5 monster is an example of a cost. Other common costs are
discarding, paying Life Points, or removing cards from play. You
have to pay any costs before you declare the activation of the
card. Even if the activation of the card is negated, you cannot get
a refund of the cost that you have paid.