WAN means Wide Area Network - otherwise known as 'the internet'. In networking terms, it's what the internet source is for a router.
Some routers support using another Wi-Fi network as their internet data connection, and this is called WiFi-as-WAN.
With WiFi- as-WAN, you can use your router to connect to a campground or marina's network, allowing all of your local area devices (laptops, tablets, streaming devices) to use that connection without having to change WI-Fi connection settings on each device.