Sean wants to know what an ARP is. He keeps getting that message when running ESET. Leo says that there's likely ARP Spoofing that is "spoofing" IP addresses that your computer allows through. It's known as ARP poisoning, and it points to malware spying on your network. But it isn't likely. More often, it's your ISP that's causing the issue. If you're using a router from your provider, it's likely in need of updating, or it's misconfigured.
Typing ARP -a from a command line will display the current IP to MAC address mapping. You can then see what MAC address is using that popup IP address and search to see what it is.