It's just a word used for emphasis. Simply meaning, "Yes, it's definitely really cold." Other examples of words used this way could be... A: "Man, I tell you, it's hot." = "It's really, really hot." (filler = "Man, I tell you") Or A: "Can you believe what happened to Mary?" B: "I know, right?" = "Right" in this example is rhetorical. This means that we aren't expecting an answer back. With this phrase and certain tone of voice, it makes "I know, right?" sound more like, "No, I absolutely can't believe what happened to her!"