What is gratuitous ARP?

The Networking on z/OS book gives this explanation for gratuitous ARP:

"The Address Resolution Protocol (ARP) Takeover provides the capability of switching OSA-Express port operations from one OSA-Express to another OSA-Express running in the same mode.

"When TCP/IP is started in QDIO mode, it downloads all the home IP addresses in the stack and stores them in each OSA-Express feature to which it has a connection. This is a service of QDIO architecture and occurs automatically only for OSD channels.

"If an OSA-Express feature fails while there is a backup OSA-Express available on the same network ID, TCP/IP informs the backup OSA of which IP addresses (real and VIPA) to take over, and the network connection is maintained. The takeover is effected by something called a gratuitous ARP. A gratuitous ARP is an unsolicited ARP response. All hosts on the LAN segment that receive this gratuitous ARP will update their ARP cache with the new MAC address for the backup OSA."