repmgrd is a daemon which runs on each PostgreSQL node, monitoring the local node, and (unless it's the primary node) the upstream server (the primary server or with cascading replication, another standby) which it's connected to.

repmgrd can be configured to provide failover capability in case the primary or upstream node becomes unreachable, and/or provide monitoring data to the repmgr metadatabase.

From repmgr 4.4, when running on the primary node, repmgrd can also monitor standby disconnections/reconnections (see Monitoring standby disconnections on the primary).