A standby instance is a copy of the active database instance which you update regularly. To update the standby instance, use the log backups of the original instance. The standby instance is always in the ADMIN operational state.
If the original instance breaks down, you can start the standby instance immediately, thereby continuing work without long downtimes. Depending on the configuration that is selected, you can start the standby instance in a state that corresponds to the state of the original instance at a point in time in the past.
Original Instance and Standby Instance
Starting the Standby Instance as an Active Instance
Replication and High Availability