In the speed control model, can you elaborate what each variable means?
Sure. The speed control model is described in Lecture 1, Slide 10. We are first given a differential equation describing our system: Mass*Acceleration = the sum of our forces. Here our forces are -av, a resistive force (friction) proportional to the velocity; the force from the engine (which we are controlling); and the force of the hill, which acts as our "disturbance" and can keep Bob from going his desired speed.
The force of the engine is k_p multipled by the error, or the difference in the desired velocity from the current one. The letter 'k' is usually used to denote gain terms, while the underscore p signifies that it is a gain /proportional/ to the error.
Finally, the graph shows that v_ss (the steady state velocity of Bob) approaches the desired velocity for a sufficiently large gain term.