Federal government of the United States
The President is in charge of many departments that control much of the day-to-day business of government. For example, the Department of Commerce makes rules about trade and business. The President chooses the heads of these departments, and also nominates judges at the federal (nation-wide) level. However, the United States Senate, part of the legislative branch, must agree with all of the people the President chooses.  This is another check on the President's power, so he cannot just fill these departments and courts with people that will always agree with him.