A thrush is a small, sweet-voiced songbird. In this particular novel, there's "an enormous thrush, nearly coal black, its pale yellow breast freckled with dark spots" (11.30). This bird proves to be extremely useful: he cracks a snail loudly and repeatedly against the sealed side door when it's Durin's Day and Thorin needs to use his key in the keyhole that's about to be revealed by magic.
The thrush also overhears Bilbo talking about Smaug's weak point. He brings this important news to Bard, who, as a descendant of the Lords of Dale, is able to understand the speech of this particular kind of bird – convenient, and we'll take it. Last but not least, the thrush brings an ancient raven, Roäc son of Carc, to Thorin. Thorin can understand raven speech, and the ravens become another way for Thorin to get news of what's going on outside of the Lonely Mountain. We don't know why the thrush is so interested in helping out Thorin & Co., but we certainly can't argue that he does.