Excited for you! I'm going from 1x 12tb USB drive to 4x internal 18tb drives. I'm building the NAS from scratch and keeping my other server for its current services (mostly Plex). My parts have been defective though, so it's all just sitting waiting for a replacement mobo.
It's a data maintenance feature that amends data in storage pools that are incorrect or incomplete. It works on BTRFS volumes or RAID 5/6 storage pools. It's scheduled to run monthly on my NAS. I guess it started now as I upgraded my drives from 4x4TB to 4x18TB.
