ARP

Address Resolution Protocol

discover

If an ip is not found in the /etc/network/arp/arp.cache file a ARP_DISCOVER message is sent. This message contains the ip address that is it is trying to resolve.

respond

If another computer has sent a ARP_DISCOVER and the containing ip is the same that is saved under /etc/network/interface the pc will send a ARP_RESPOND. The origin of that message will then tell the computer that has sent the ARP_DISCOVER signal where to send packages for that ip.