ARP stands for Address Resolution Protocol.
IP packets are sent between IP addresses, but the Ethernet frames in which they are packaged are sent between MAC (hardware) addresses. Therefore the MAC address of the destination IP address must be known before an IP packet can be sent onto an Ethernet network.
The Address Resolution Protocol (ARP) is used to obtain MAC address information. FreeRTOS+TCP (like most if not all IP stacks) stores IP address to MAC address mappings in an ARP table (sometimes called the ARP cache). ARP is handled automatically by the TCP/IP stack.