Let me offer some more information, because we get this a lot, so I'll be more verbose and offer it for later Googlers who come across the post.
Ethernet does not use IP address, it uses MAC addresses. ARP is the protocol that handles which IP address goes with which MAC address. If you have, say, a main router, and a "wireless extender" on your network (this is a common situation), then if you suddenly go from the main router with your (say) phone to the wireless extender, that will entail a MAC address change, which will cause ARP to announce that change, since your device is now reachable via a different ethernet path, if you follow. A lot of IoT and Apple devices, as
@nojohnny101 said, induce this kind of dynamics on your LAN, in addition to, like I said, range extenders.
Such ARP address changes though, in certain not-very-frequent circumstances, are also induced by certain types of hackers redirecting stuff. This is why you get such warnings.
99.78% of the time however, there is absolutely nothing to worry about when you see these messages.
Accordingly: I myself turn it off (hackers, have fun trying to get to my LAN!!) by adding the appropriate sysctls, in System->Tunables->Add Tunable->sysctl:
Code:
net.link.ether.inet.log_arp_movements=0
and, probably this related one as well:
Code:
net.link.ether.inet.log_arp_wrong_iface=0
To be clear, the "variable" is the thing to the left of the equals sign, and the "value" is 0 for each.