How can ARP be used to launch attacks?

ARP (Address Resolution Protocol) can indeed be exploited to conduct various types of attacks, most notably ARP spoofing/poisoning. Gratuitous ARP is a special kind of ARP message which is used by an IP node to announce or update its IP to MAC mapping to the entire network. A hacker can abuse this by sending out gratuitous ARP messages pretending to associate the IP address of the router (default gateway) with their own MAC address. This results in traffic that was supposed to go to the router being sent to the attacker instead, thus potentially enabling the attacker to intercept, modify, or block traffic.