Fulling is what happens when you subject knitted wool to heat, water, and agitation until the fibers tighten, bloom, and bond into something denser and more durable than what you started with. It is felting's more controlled cousin.
The fabric shrinks. The stitches disappear. What remains is not what you made but what it was always becoming. Every knitter who has accidentally fulled a sweater in a hot wash knows this feeling, though usually with more swearing. On purpose, it is a transformation. Off purpose, it is a lesson.