Per the manpage:

The arp tool manipulates or displays the kernel’s IPv4 network neighbor cache. It can add entries to the table, delete one or display the current content. ARP stands for Address Resolution Protocol, which is used to find the media access control address of a network neighbor for a given IPv4 Address.

You can use the arp command to list all of the devices on the local network. It is often useful to find the identities of hidden devices on your network. For example, if you just plugged a Raspberry Pi into your local network and need to find its IP address in order to connect to it via SSH.