It can be anythingpower, toughness, something in rules text, the name of a card (Four Wishes, anyone?), collector number, and so on
Now if your foe goes, "Wasteland, sacrifice" to try and destroy my land, I can cast Stifle
The Judgment expansion set was meant to balance this, and skews towards green and white, black's enemy colors
First and foremost, this can allow a black tribal deck player to draw a significant number of cards at the cost of some life, as long as they'd been able to establish a significant board state
Cards which slot into already existing archetypes Ignoble Hierarch in Jund Ignoble Hierarch gives the Jund colors access to a mana dork of the same caliber as Noble Hierarch, and any deck from BG Infect to Goblins will be looking to try the card out
The same goes for "Oops", allowing you to respond with "Sorry"