In positive dog training a cue is a word or a signal to which your dog responds by doing something. Sit is both a cue (word or signal) and a behavior (something your dog does).
Since dogs are masters at reading body language and learning routines, we need to understand that we offer cues (often unintentionally) all the time. What does your dog do when you pick up your car keys? his leash? your running shoes? Does your dog go spastic with that "can I go, please please please can I go?" behavior? Think about it. Fido wasn't born knowing that jingling keys mean car. This is learned behavior through observation.
Why should you care? Because understanding how behaviors happen - both good and bad - can enhance your ability to change or modify those behaviors. Don't know how? Enlist the help of a positive dog trainer.