Behavior in simple terms is the visible attitude of a person. A good behavior is often rewarded whereas a bad behavior is often punished. It seems that "behavior" of a person, or from an ID perspective - learner, can be modified by providing certain conditions and then waiting for the learners' reaction to that condition. The condition that you provide to a learner is called stimulus and the response you expect from the learner is called response. Therefore, behaviorist theories provide a stimulus to the learner and expect responses based on them.