"Failover" is when you have more than one WLC or if you configure H-REAP,
When you have more than one WLC, when the WLC-A fails, the APs can swing to the next WLC that's been configured into them. The downside is during the failover there's a 5-10 seconds of lost connectivity to the WLAN while the APs search for another WLC.
When you configure H-REAP and, say, if you have only one WLC, when the WLC fails and the APs are not rebooted, the APs will function as if the WLC is still present. This works even when you have VoWLAN.