Miracle is a new keyword that allows you to cast some instants and sorceries for very little mana when you draw them.
Banishing Stroke

As you draw an instant or sorcery with miracle, if it's the first card you've drawn this turn, you can immediately reveal it. When you do so, you may cast it for its miracle cost. It doesn't matter whether it's an instant or sorcery; if you choose to cast it, you do so right away, even if it's at a time (such as your draw step) when you couldn't normally cast it. You're not required to reveal a miracle card, even if you could pay for it. You can always choose to just draw it as normal.

You're allowed to cast the first card you draw each turn if it has miracle, so if you find ways to draw cards during your opponent's turn, you get another chance for a game-breaking, miraculous moment.

Banishing Stroke 5W
Instant (Uncommon)
Put Target artifact, creature, or enchantment on the bottom of its owner's library
Miracle W (You may cast this card for its miracle cost when you draw it if it's the first card you drew this turn)