WiFi Assist Fallback can be found in Guardian Settings > Advanced
This advanced feature allows the user to enable their device to maintain two concurrent VPN connections at the same time, allowing for faster network switching.
In other words, this is mainly to assist maintaining a persistent connection when you are a user that frequently switches between networks such as moving between buildings and changing from Wi-Fi to Cellular. Normally, the connection terminates and then the built in fallback mechanism automatically re-establishes the connection, but this can take a couple of seconds to fully connect and reestablish a secure connection. The Wi-Fi Assist allows for two separate tunnels to be maintained so that there is less interruption when switching between networks.
While your device is connected to WiFi the cellular connection is still maintained in the background. This is done in order to still be able to receive phone calls, or transition back to WiFi once the device becomes within reach of the WiFi signal. If a WiFi network is connected, the connection is simply preferred by the system since it is more likely to provide faster network access.
This option should only be used by people who regularly transition into and out of a space where the device may be able to reconnect to a known WiFi network or lose connection to a known WiFi network. Most users will see no benefit by enabling the switch or may even observe reduced battery life.