ARP(ADDRESS RESOLUTION PROTOCOL) is a network layer protocol which associates the physical hardware address of a network node(commonly known as a MAC ADDRESS) to its ip address. now an ARP creates a table known as ARP CACHE/TABLE that maps ip addresses to the hardware addressess of nodes on the local network.

if based on the ip address it sees that it has the node's mac address in its ARP TABLE then transmitting to that ip address is done quicker because the destination is known and voila network traffic is reduced.