Reknit can regenerate an artifact, creature, enchantment, land, or planeswalker. If you regenerate a noncreature permanent, the next time that permanent would be destroyed that turn, instead tap it. If that permanent had any damage on it (because it had been a creature earlier in the turn), that damage is removed.

Regenerating a planeswalker will prevent it from being destroyed as a result of a "destroy" effect (like the one from Reaper King). However, it won't prevent that planeswalker from being put into its owner's graveyard when its loyalty becomes 0.