A signal can be anything perceptible. It may be audible, for example your voice or a whistle, or visual, for example a hand signal or a flashing light. A signal is used in training to direct the dog to do a particular action.
The signal is sometimes also called a command or cue. The dog has to learn the meaning of the signal and to do the appropriate action on cue, i.e. in response to the signal.
According to the *STAR* system, S for Signal is a separate training task from getting the behaviour and from the aspect of reinforcement or maintaining motivation in the long term. It is called the Stimulus Control stage of training.