Cascade triggers when you cast a spell that has it. You exile cards from the top of your library until you exile a nonland card that costs less than the cascade spell. You may cast that card without paying its mana cost, then put all exiled cards on the bottom of your library in a random order. Some cards have cascade twice, doing it all over again.
Cascade matters because it hands you a free spell and digs deeper into your deck. It can chain: a cascade spell might flip into another cascade spell, and the value keeps stacking. Because the card you hit is random, there's luck involved, but casting something for free is a powerful tempo swing.
