Anyone who watches a poker game is said to be a railbird. The typical railbird is brimming over with advice, guidance, and is cheerful, especially if he is hoping for a stake from one of the seated players. Railbirds who cheer on a specific player are said to "sweat cards", a term that connotes the empathy felt by the railbird as he lives vicariously through a real poker player.
A railbird is someone who watches poker games. They get their name because generally observers of poker games must stand behind a rail. Sometimes, railbirds are poker players who are broke and want to get back into the action.
Simply put, a railbird is one who stands by the railing. When someone wants to just see a poker game, they are allowed to do so from behind a railing. And someone who constantly does this is called a Railbird.