Linux is a very popular choice for server applications, but many businesses require extremely high availability. For example, phone companies require "five nines" of availability. That's 99.999% uptime.
Sure, my desktop PC might go a month or two between reboots. But how do you ensure that your Linux-based application is available ALL the time?
"Linux HA" is a set of tools that allows you to cluster several Linux systems together to provide a very high level of availability. When one system is not responding, the next one takes over.