ARP stands for Address Resolution Protocol

Arp manipulates or displays the kernel’s IPv4 network neighbour cache:

arp -a

This show the IP addresses and MAC address of the devices your computer knows about on the network.

As I understand it…

A computer finds devices on the network by sending a broadcast saying who has this IP address. The device with that IP address will reply with it’s MAC address. This information is cached and is what you see when you run arp -a