An ARP cache[1] is a collection of Address Resolution Protocol entries (mostly dynamic), that are created when an IP address is resolved to a MAC address (so the computer can effectively communicate with the IP address).
[2] An ARP cache has the disadvantage of potentially being used by hackers and cyberattackers (an ARP cache poisoning attack).
An ARP cache helps the attackers hide behind a fake IP address.
[1] Beyond the fact that ARP caches may help attackers, it may also prevent the attacks by "distinguish[ing] between low level IP and IP based vulnerabilities".
[3] This computer networking article is a stub.