A Router Solicitation message is part of the ICMP protocol. It is used heavily with IPv6 and NDP.
When an IPv6 device on the network needs to learn more about the local portion of the network, it can send out an RS message.
This typically happens when the interface is first enabled, or configured with an IPv6 address. It can learn things such as the link’s prefix, and find routers on the network.
A router that receives this message will send back a Router Advertisement message as a response. This message includes the information that the client asked for.