vtctld is a Vitess server component that can perform various Vitess cluster- and component-level operations on behalf of an administrative user. You can interact with vtctld via a web UI, or via an gRPC interface using the vtctlclient CLI tool. The web UI allows you to browse the information stored in the Topology Service, and can be useful for troubleshooting or for getting a high-level overview of the cluster components and their current states.
Some of the administrative actions vtctld can perform include: reparents (failovers), backups, sharding, shard splits, resharding, and shard combines.