Simple definition
A SQL Server failover cluster instance provides instance-level availability using Windows clustering and shared storage.
Purpose and use
A FCI, Failover Cluster Instance, provides a SQL Server instance that can run on different Windows cluster nodes according to cluster state.
How does it work?
The cluster coordinates instance resources, network identity and supported storage; normally one node owns the active instance at a given moment.
Example and verification
Test failover by verifying SQL service, storage access and actual application reconnection.
Points to watch
A FCI does not inherently create independent copies of every database. Shared storage dependencies require their own protection.
In short
Assess instance and data availability together with backups and cluster dependencies.