Hi charmax001
Thanks for your question.
FCI stands for Failover Cluster Instance, and HA stands for High Availability. These are two concepts related to ensuring the availability and reliability of SQL Server databases.
FCI stands for Failover Cluster Instance, which is a feature of SQL Server that provides high availability by clustering multiple servers together to act as a single instance. This means that if one server fails, another server in the cluster can take over and provide continued service.
HA stands for High Availability, which is a broader term that refers to any solution that provides continuous availability of a service or application. In the context of SQL Server, HA can be achieved through various methods, including FCI, Always On Availability Groups, and database mirroring.
Please watch the YouTube Video https://youtu.be/mN18npFVFmE from timeline 1:23 which explain about FCI (What is Failover cluster) and also Talks about availability group from timeline 4:28.
Also sharing you the Microsoft community blog about "An overview of High Availability and Disaster Recovery solutions available for SQL Server"
https://techcommunity.microsoft.com/t5/core-infrastructure-and-security/an-overview-of-high-availability-and-disaster-recovery-solutions/ba-p/370479
Let us know if you have any additional questions on those lines.
Thank you!