When something subcools, it becomes cooler than it was before.
Subcooling is usually a good thing because it means something is getting cooler.
Examples:
The water subcools and becomes ice.
He subcools the drink before he drinks it.
She subcools the room by opening the window.
The ice cream subcools in the freezer.
The superhero subcools the hot lava with his freeze ray.
The wizard subcools the potion to make it extra cold.