Simple definition
SQL Server Always On availability groups replicate databases across instances for high availability and disaster recovery.
Purpose and use
An Always On availability group maintains SQL Server database replicas for availability or disaster recovery.
How does it work?
A primary replica receives writes and sends log records to secondaries using configured synchronous or asynchronous behavior.
Example and verification
Before failover testing, inspect synchronization, permitted failover mode, relevant quorum and the application's intended connection endpoint.
Points to watch
Asynchronous replication can lag, and forced failover can lose data. A group does not replace backup or automatically replicate every instance-level object.
In short
Recovery depends on mode, replica state and application dependencies actually tested.