The thrush, another helpful bird living near the Lonely Mountain, brings the ancient raven Roäc to speak to Thorin. Apparently, Thorin can understand raven speech, and used to have frequent chats with Roäc's father Carc before Smaug came. Roäc remembers the old days when the dwarves lived under the Lonely Mountain, and is willing to help Thorin out. He brings Thorin the news that Smaug has been killed by Bard (yay!). But he also lets them know that the humans and elves are gathering armies to come and see what has happened to Thorin, and if there's any treasure up for grabs (boo!).
Roäc advises Thorin to try and negotiate with Bard, but Thorin refuses. Instead, he asks Roäc to send his ravens with all of this news to Thorin's cousin, Dain. Thorin wants Dain to bring his dwarf armies to support Thorin in battle against the humans and elves. While Roäc doesn't think much of this idea, he still agrees to carry Thorin's messages for him.
Ravens play a vital role in Norse mythology as messengers. The great god Odin carries two ravens on his shoulders, Huginn and Muninn. These ravens fly out during the day to find information from Odin and then return at night to tell him all that has happened in the world. So Roäc's role as a bearer of news seems to be thoroughly grounded in mythological sources.