Complex interactions encompass everything that simple keyframe animation can't. For instance if you want the user to be able to click and drag to move or rotate an object you will require a sensor. Likewise, if you need to keep track of an object's state (eg. whether a door is open or closed) then a small script is required.
Important: Comprehensive coverage of interactions and scripting are beyond the scope of this one semester introductory course. The above exercises are designed simply to give you a taste of what can be done. For your own projects limit interactivity to only that which can be accomplished with keyframe animation.