Stimulation spurs an organism to act. It can also be any specific event that causes someone to do something.
Have you ever had a teacher who made you excited about a subject you didn't like before, causing you to study harder and learn more? That teacher must've provided stimulation: through the teacher, you were motivated to give more effort to the class. Like a stimulant, stimulation causes things to happen. A doctor hitting your knee is a type of stimulation that makes your leg jerk up. In zoos, animals are provided with food-based stimulation to make them exercise.