Reparenting is the process of changing a shard’s primary tablet from one host to another or changing a replica tablet to have a different primary. Reparenting can be initiated manually or it can occur automatically in response to particular database conditions. Vitess supports two types of reparenting: Active reparenting and External reparenting.

• Active reparenting occurs when Vitess manages the entire reparenting process. There are two types of active reparenting that can be done: Planned reparenting and Emergency reparenting.
• External reparenting occurs when another tool handles the reparenting process, and Vitess just updates its components to accurately reflect the new primary-replica relationships.

You can read more about reparenting in Vitess here.