Keep another server ready
An online shop might run on two servers. If one stops working, the other can keep the shop open. Having the second server is redundancy; sending customer requests to it is failover.
In an active-passive setup, one server waits to take over. In an active-active setup, both already handle requests. Either way, they both need current data and a way to spot a failure.
Spares must be separate
Two servers can fail together if they share a power source or depend on the same database. Teams test the switch to the spare and check that customers can still use the service. Extra servers alone do not guarantee uptime.