Stimuli is anything that can be detected by hearing, seeing, feeling, taste, etc. In other words, the senses. This can range from sounds, food, drinks, smells, touches and visual signals including clickers, and snapping sounds made by the leash or collar also including whistles. I am not a fan, although this training works, only because you might not always be around your dog to give it a pat, you might not always have an electronic collar, and you might forget or run out of treats. A pat on the head and some scratching, rubbing and petting along with praise is always available.