What is failover?
Failover is a shorthand for a backup server that lets effective service continue without any noticeable difference on the front end. It’s used for ‘high-availability’ systems that must continue to run in order for operations to continue.
When set up properly, there should be no reduction in traffic, no downtime, and no change for the user. However, depending on the exact machines in the network, there might be fractional reductions in availability for exceptionally high-load systems in the few microseconds it takes to switch over to the backup system.