High availability (HA) is the ability of a system to operate continuously for a designated period of time even if components within the system fail. A highly available system eliminates single points of failure. This is accomplished by including redundant components that serve as backups that can assume processing if failure should occur. In information technology (IT), a widely held but difficult-to-achieve standard of availability is known as five-nines availability, which means the system or product is available 99.999% of the time.

High Availability describes systems that are dependable enough to operate continuously without disruption due to the underlying hosts failing.

“Availability” includes two periods of time: how much time a service is accessible, and how much time the system needs to respond to user requests. High availability refers to those systems that offer a high level of operational performance and quality over a relevant time period.

To reduce interruptions and downtime, it is essential to be ready for unexpected events that can bring down servers. At times, emergencies will bring down even the most robust, reliable software and systems. Highly available systems minimize the impact of these events, and can often recover automatically from component or even server failures.

If your server is deployed in a location with 3 of more physical hosts in a cluster, then it is very likely your server has HA functionality.
You should not notice the feature as it should run silently in the background.
If you notice the host of your server randomly changes without downtime recorded, this can be a sign that you are in a HA cluster.

• Eliminates single points of failure. A single point of failure is a component that would cause the whole system to fail if that component failed. For example, if a business uses only one server to run an application, that server represents a single point of failure. Should the server fail, the application will be unavailable.
• When a component fails, it is possible for data to be lost if data protections have not been put into place. An HA system includes the mechanisms necessary to avoid or minimize data loss or disruption during system failure.
• Failures or faults must be immediately detected and acted upon.
• If a system component fails, a similar component must be immediately available to take over for the failed component. For this reason, the system must include redundant components and ensure reliable failover, which is the process of switching from one component to another without losing data or affecting performance, availability or operations.