Local access mode gives you the ability to access the device directly on its local network to configure its communication settings. Your device will start up a web interface that you can connect to, monitor the current status, and make any changes to your device's settings.
These settings can be used to:
Connect your device to an internet connection so that is becomes available for use.
Configure the second LAN port to connect to a different network that hosts your camera(s).
Out of the box, your device has default networking settings that in many circumstances are enough to get your device connected and find your cameras.
However, if you find your device isn't appearing in the web app, or you need to configure Wi-Fi or LAN then you'll use local access mode to check and configure your device's network settings.
To use local access mode, it first must be enabled (either manually or automatically) and then you can connect to your device to configure it.