High availability is a way of computing that allows for the continuation of services even when other services fail. It is designed to avoid loss, ensure availability, and a high percentage of uptime. Systems typically have some sort of redundancy in place to prevent a loss of uptime, meaning that there is not a single point of failure within the system.
Within computing, the term availability refers to the uptime and reliability of the service or system that you’re using. Availability is often measured in terms of percentages. Typically within the hosting industry, you will see 99.999% uptime (which is around five minutes of downtime) as the measurement meaning that 100% is no downtime whatsoever. This is achieved typically with a load balancer along with a few other components like virtual machines, clusters, and hardware.
Load balancers are a single service and therefore a single point of failure and cannot guarantee high availability, you will need to have something in place to detect when a failure happens so that action can be taken when one part of the system goes down.