Address Resolution Protocol (ARP) cache is a table where router or computer is keeping Layer 2 MAC addresses to Layer 3 IP addresses mappings which hw resolved lately. Without MAC to IP mapping the device would not be able to build and send IP packets.
Device discovers the MAC to IP mapping using ARP protocol Broadcast requests on the local subnet and saves the mapping in the ARP cache. If a device wants to send a packet to a neighbour, and it doesn’t have a corresponding MAC address, it sends out a broadcast ARP request packet. This broadcast packet reaches every device on the local subnet. Everybody will receive that ARP request packet but only the one device with the IP address mentioned in ARP broadcast request packet responds back with an ARP response packet to complete the process.