The horizontal blue line is the setpoint; the ragged orange line is the measured variable when there is no control. The white (cream-colored) bar indicates that there is a delay between measuring the variable and taking control action. The delay could result from several factors. Let us assume that the analysis of a sample from our process takes a little time.
Look at Bar #1. We take the sample at the start of the bar and apply corrective action at the other end of the bar. Everything looks pretty good because the deviation from the set point is about the same.
At Bar #2, the corrective action is taken after the variable has moved to the other side of the set point. We simply make things worse by applying correction at the wrong time.
At Bar #3, you might think that you are ok because the variable has returned to about where it was at the start of the bar. These times when you are lucky and apply the corrective action so that it helps are overshadowed by the times when you make things worse. In fact you may have over corrected to the point where the variable is way worse than it would have been with no control, and the control action is a haphazard mess.