Congress passes laws. The Constitution states it's the President's job to “ensure those laws are faithfully executed.” So, Presidents often use Executive Orders to direct federal workers on how to enforce existing laws, sometimes changing direction during times of war or other emergencies.
An executive order is a written statement that the President issues to “direct or instruct the actions of executive agencies or government officials, or to set policies for the executive branch to follow.” It is signed by the President, approved by the Office of Management and Budget and the Attorney General