WardWard is a triggered ability. Ward [cost] means “Whenever this permanent becomes the target of a spell or ability an opponent controls, counter that spell or ability unless that player pays [cost].”
Some ward abilities include an X in their cost and state what X is equal to. This value is determined at the time the ability resolves, not locked in as the ability triggers. (CR 702.21)—Pay 3 life.
Whenever The Meep attacks, you may
sacrificeTo sacrifice a permanent, its controller moves it from the battlefield directly to its owner’s graveyard. A player can’t sacrifice something that isn’t a permanent, or something that’s a permanent they don’t control. Sacrificing a permanent doesn’t destroy it, so regeneration or other effects that replace destruction can’t affect this action. (CR 701.21) another creature. If you do, creatures you control have base power and toughness X/X until end of turn, where X is the sacrificed creature's mana value.