In order to produce a control input (engine force in this case) that alters the value of the controlled variable (speed), one computes the error (difference) between its desired value and its actual value. Here, the error would be e = vdes - v. After computing the error, one uses it to produce the control input. In general, the control input can be represented as f(e,.), where f is a general nonlinear function and . represents other variables/ parameters. Here, f has been chosen to be a simple linear function, i.e., f(e,.) = ke = k(vdes-v). This type of controller is called a proportional controller. You will encounter other types of f as the class progresses.