Sometimes, we want higher uptime and a higher throughput with our internet connection, and when we do, multiple WAN interfaces providing multiple internet connections is a way of doing this. Before you implement multi-WAN, you must already have a working LAN and WAN interface. Additional WAN interfaces are referred to as OPT WAN interfaces, and pfSense can support several WAN interfaces.
In order to fully take advantage of a multi-WAN setup, however, you should have separate internet connections. Ideally, you should have separate internet connections from separate internet providers, because there is a higher likelihood of a connectivity issue between your network and a provider affecting all connections to that provider than there is that a connectivity issue would affect multiple providers at the same time. And of course, if you have two connections to the same...