Multi-WAN and bonded circuits are often confused.
Multi-WAN means you have one or more WAN connections that pfSense can use. This is very common in a business setting where you have one Internet connection that for example can be the "main" WAN connection and another, less expensive link as a "secondary" or "failover" link. If you have a failover gateway group created, traffic can go from using the main connection to the failover if the main link goes down. Gateway groups can also be used load balance Internet connections by balancing traffic across the two links. This is different from bonding links together. Balancing traffic mean each connection would carry traffic exclusively versus a bonded pair that presents all of the bandwidth as a single connection.
https://doc.pfsense.org/index.php/Multi-WAN
https://doc.pfsense.org/index.php/Gateway_Settings