What does Rail mean in poker and casino games?

Rail is the area around the table, or the viewing line where spectators stand and watch.

How is it used?

In poker, the term is especially common for spectators watching an expensive or important game from the outside. That is where expressions like on the rail come from. In a broader gambling sense, rail can also mean the outer edge of the table or the space where people stand without taking part in the hand. In simple terms, it is a term about the space around the game, not about how the betting works.