ARP (Address Resolution Protocol) is used to map layer two information to layer three information. An example of where we use this is Ethernet. When one host wants to send an IP packet to another host, it has to know the destination MAC address which is unknown. The first host will then broadcast an ARP request which is received by all other computers. The host that has the MAC address that is requested will respond with an ARP reply, supplying its MAC address.
The first host now knows the destination MAC address and will continue by forwarding the IP packet.