We, as sinners, have wracked up a debt we cannot pay. But God, in God's great mercy, mints a coin of infinite value. Not a a trillion-dollar coin, but a coin of infinite value! This coin is Jesus Christ. And God deposits this Coin in the Treasury of Heaven which can now--because of its infinite value--cover all our sin-debts.
Praise be to God!
Silliness aside, this metaphor is actually closer to Anselm's substitutionary thinking than the penal substitutionary model that later eclipsed it. Specifically, the trillion dollar coin metaphor doesn't hinge on God's wrath--a crime and punishment trope--but on the infinite value of Jesus that can cover any and all debts.
(And apologies to all my Non-USA readers if you've not kept up with our trillion dollar coin debate. A quick Google search of "trillion dollar coin" should catch you up.)