Final answer:
A president issues an executive order to set policies for the executive branch or reorganize it within existing law, which has the binding force of law and does not require Congressional approval.
Step-by-step explanation:
The power a president uses to issue rules that set policies for the executive branch or reorganize it within the constraints of existing law is called an executive order. Executive orders are directives issued by the President that have the force of law and are used to manage the operations of the federal government. These orders can direct the way legislation is implemented and are treated by courts as equivalent to laws. Famous examples include Lincoln's Emancipation Proclamation and Truman's desegregation of the armed forces.
Executive orders allow the President to take decisive action without the need for Congressional approval, thus enabling quick responses to situations when necessary. While they can be challenged by the courts and Congress, many executive orders stand unopposed and become a significant part of the President's legacy and impact on domestic and foreign policy.