It lets you equip more charms than you'd usually be able to.
Charms take up a certain amount of notches, and throughout the game you get more notches
If you have the Grimm troupe dlc the maximum is 11, luckily the switch version comes with it
Charms take up a certain number of notches, so if that number exceeds the amount of notches you have access to you can't equip the charm, and will have to equip one that takes up less notches.
UNLESS you have the void heart, which let's you equip that last charm even if it exceeds the maximum charm notch amount
... However you still can't equip more after that last charm
