In the control task, you monitor the work being done. You compare the actual progress to the plan. You verify that the organization is working as you designed it.
If everything is going well, you do not need to do anything but monitor. However, that seldom happens. Someone gets sick, the database sort takes longer each iteration than projected, a key competitor drops their prices, a fire destroys the building next door and you have to evacuate for several days, or some other factor impacts your plan. The control step now dictates that you have to take action to minimize the impact and brings things back to the desired goal as quickly as possible.That means going back to the planning stage and adjusting plans. It may require a change in the organization and you will have to re-direct everyone toward the new goals. Then you control the new plan and adjust if needed. This cycle continues until you complete the task.
Here are some additional aspects of Monitor and Control:
Keeping Score Isn't Winning
There is no point in monitoring if you don't take action based on the information. Don't just keep track. Make sure you are measuring the right things and then take appropriate action to fix whatever problems the measurements point out.