Plot lets you pay a cost and exile a card from your hand. You can only do this as a sorcery: during your main phase on your turn, with an empty stack. The card waits in exile. On a later turn, you may cast it as a sorcery without paying its mana cost. You pay once, wait, then get the spell for free.
Plot matters because it spreads a card's cost across turns. You can use mana on an early turn to set up, then later cast the card while keeping mana open for other plays. It also lets you cast multiple spells in one turn without needing extra mana. The trade-off is that you must plan ahead, since you cannot plot at instant speed and you must wait for a later turn to cash in.
