It could be the IP address of the device that's being powered, or some other IP address, actually.
When the pings fail, the port will be power cycled. Here's some information from the EdgeOS User Guide about ping watchdog:
PoE Watchdog
PoE Watchdog is only for PoE-enabled ports. It configures
the device to continuously ping a user-defined IP address
(it can be the Internet gateway, for example). If it is
unable to ping under the user-defined constraints, then
the device will automatically turn off PoE on the port,
and then turn it back on. This option creates a kind of
“fail-proof” mechanism.
PoE Watchdog is dedicated to continuous monitoring of
the specific connection to the remote host using the Ping
tool. The Ping tool works by sending ICMP echo request
packets to the target host and listening for ICMP echo
response replies. If the specified number of replies is not
received, the tool reboots the device.
• Enable Watchdog Enable the use of PoE Watchdog.
- IP Address To Ping Specify the IPv4 or IPv6 address
of the target host to be monitored by PoE Watchdog.
- Ping Interval Specify the time interval (in seconds)
between the ICMP echo requests that are sent by PoE
Watchdog. The default value is 15 seconds.
- Startup Delay Specify the initial time delay (in
seconds) until the first ICMP echo requests are sent by
PoE Watchdog. The default value is 300 seconds.
The Startup Delay value should be at least 60 seconds
as the network interface and wireless connection
initialization takes a considerable amount of time if
the device is rebooted.
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Basically, it's a fail safe device. What's the first thing you try when something stops working right? You power cycle it.
Ping watchdog automates that. If the device you're powering is locked up and can't respond to pings, ping watchdog will detect that, and turn off/wait/turn on power to that device. That *may* make it work again.