The Bestiary tracks your progress for each monster you hunt. By killing enough of a creature, you complete its entry and earn Charm Points.
Charm Points can be spent to unlock Charms, and then assigned to specific monsters. Once assigned, the charm can trigger special effects while fighting that creature, making it easier and more efficient to hunt.

The Bestiary overview showing monster progress and completed entries.

The Charms assignment window where you apply a charm to a specific monster.