Mystery boxes are boxes you can buy that will randomly drop certain items configured for that box. There are different tiers of boxes that range in the value of items dropped.
All mystery boxes are purchased with an in-game currency known as Spirit Points. No purchase is necessary to obtain Spirit Points. You can earn SP (Spirit Points) by completing tasks that get you XP, such as commenting on art, forum posts, and participating in PDARPG art challenges. You can also get Spirit Points by becoming a Patron on Patreon.