Internet Failover is a relatively new term, and in simplistic terms means you have a redundant Internet service in case one system goes down (when one goes down, the other ISP is ready to provide seamless internet). While this is typically quite cumbersome to setup and manage, Simplifi has failover configured out of the box. To determine your best failover instance, please search under "Modem Failover", "Router Failover" or "Router Failover with Firewall".
In Technical terms, Every five seconds we transmit ICMP echo packets to measure packet loss and latency on each interface. If the packet loss and/or latency exceeds our thresholds for three consecutive health checks, we mark the interface as down and will re-route all packets through the secondary interface. We continue monitoring the packet loss and latency on a "down" link, and when 8 consecutive health checks are within acceptable thresholds, we mark the interface as "up" and will re-route all traffic to use the primary. For the cellular link our thresholds are 1000 milliseconds latency and 20% packet loss. For wired WAN we use 5000 milliseconds latency and 40% packet loss, since some customers use high-latency and high-loss networks like satellite.