What is an Always On Availability Group in SQL Server, and what problem does it solve?

An Always On Availability Group is a high availability feature that keeps a copy of one or more databases synchronized across multiple SQL Server instances, so if the main server fails, another server already holding an up to date copy can take over quickly, minimizing downtime for your applications.

-- Simplified concept: primary replica and secondary replicas
-- stay synchronized, with automatic failover configured
CREATE AVAILABILITY GROUP MyAG
FOR DATABASE MyDatabase
REPLICA ON 'ServerA' WITH (...), 'ServerB' WITH (...);