This SSID and password will be tried when NAM can not connect to main WiFi network. If configured - NAM will try to connect once after it has failed connecting to main network.
This feature can be useful if you manage many sensors and some of them are placed in different networks you don’t control. Configure fallback Wi-Fi for your lab and when sensor get to you for some maintenance then it will connect automatically - no need to reset Wi-Fi config. Then after it is placed in regular place - it will connect to main network.
Just leave SSID empty to disable feature.