Morph lets you cast a card face down as a 2/2 creature for three generic mana. If the face-down permanent has morph, you can later pay its morph cost to turn it face up, often revealing a much more dangerous creature or triggering another effect.
Brine Elemental famously combines with Vesuvan Shapeshifter to repeatedly deny opponents untap steps, while Trail of Mystery and Secret Plans reward building an entire deck around face-down creatures. Exalted Angel is a Premodern format all-star with the morph mechanic.
