When an IPv6 device on the network wants to learn more about the local portion of the network, it sends out a Router Solicitation message. This is so it can learn things such as the link’s prefix, and find routers on the network.
The router sends back a Router Advertisement as a response, which includes the information that the client has requested.
Routers periodically send out these RA messages. This helps to make sure that all IPv6 devices on the link have been refreshed with the latest information.