ARP (Address Resolution Protocol) is a data link layer protocol and is used to find MAC (Media Access Control) Address from IP address. MAC address which works on host link layer or data link layer is also known as hardware address and stored in NIC and cannot be changed. IP address works in upper layer, network layer of layered architecture.
ARP request message contains senders MAC address and IP address which are also known as SHA (Sender hardware Address) and SPA (Sender Protocol Address) respectively. This message also contains target IP address (Target Protocol Address). This message is broadcasted so that every receiver knows the MAC,IP binding for source in its cache, This message is also known as ARP announcement.
And the target machine which has IP address as TPA in ARP request sends back ARP reply message to update MAC ,IP cache of source by saying that this is my IP address and my MAC address is this.
This task is performed in OS startup because IP address can be changed in LAN and even NIC card can be changed. MAC IP binding cache is maintained so it is not needed to perform it for every packet.