WAN = Wide Area Network. That’s a fancy way to say the internet. That’s it.
ping is a tool that network engineers use to help determine whether a device on a network is working or not. It’s the simplest test you can run to determine connectivity – sort of the equivalent of knocking on a door and listening for a “Who’s There?”. If you don’t receive a response from ping, there’s probably a very simple reason such as a networking cable becoming unplugged.
A router is said to be WAN Pingable if it responds to pings coming from other the internet. This is a great service to help determine things such as whether or not a server is operating correctly.
But there’s another side to ping. Hackers often use it to test random addresses on the internet to see if there’s something there to potentially break into. Sort of like calling random phone numbers to see if someone is there to see if you want to break into a house. If your router does not respond to a ping, you’ve taken an important step to becoming invisible to the bad guys.
Now the truth is, ping is an important tool for large corporate servers, but it’s hard to imagine why a home router would ever need to respond to a ping coming from the internet. So, if your router allows it, turn off WAN pings.