If you are taking AP Government and Politics, this is one of the foundational items you must understand. You will be asked, on the exam, how one branch can have influence over the other.
The Executive Branch (President) has considerable control over the other two by using something called "Discretionary Authority." The word 'discretionary' comes from the Latin root which means "to separate, distinguish, or decide." In essence, the Executive Branch makes Rules about Legislative Acts and SCOTUS decisions by way of Discretion, or decision-making.
Is it widely accepted that the Executive Branch 'enforces the law.' However, what does that mean? It means the Bureaucracy created by the Executive Branch was designed to create regulation (rules) about laws it must enforce. For example, if Congress passes a law (which the president signs) about something - let's say education- the Executive Branch must enforce it. How they do it is up to the President who is in office at the time and his/her political outlook. If the President is more a champion of local rule, then the Department of Education will interpret the law, and use its Decision-making (discretionary authority) to make a rule on how the law should be carried out. If the law says 'government money will only be given to states who show student improvement in math' then the DOE will create a rule about this in regard to the giving of money to the state departments of education.
Likewise, if the SCOTUS makes a decision about something - let's say Brown v Board of Education' the Discretionary Authority of the DOE in the Eisenhower Administration created rules (regulations) on public school districts forcing them to allow African American children into all-White schools. Eisenhower used the US Military to enforce the ruling in that case. He could just as easily do nothing, and that is the crux of Discretionary Authority. It is the decision-making power the President/bureaucracy has over the other two branches of government.
