The president is the chief executive of the country. He heads the executive branch of the government. Millions of people work for the various agencies and departments of the executive branch. They include the secretary of defense, park rangers in national parks, and the letter carrier who delivers your mail.
In name, the president is in charge of these millions of workers. In reality, he appoints their supervisors. They make sure that the many departments do their jobs. The executive branch includes the Executive Office, the Office of the Vice President, and the cabinet departments. It also includes more than 100 agencies, boards, commissions, and committees.
Here is a tiny sampling.