Answer:
The Separation of Powers is central to the US system of government.
Step-by-step explanation:
The experience of the American colonists at the hands of the British was such that they were determined to guarantee that no single branch of government would become too powerful after they won independence.
There are three parts of government, and this is how they operate, in theory.
Firstly there is the legislature which in the USA is Congress. The legislature makes laws. To try and guarantee fair representation all states have 2 senators, and in The House of Representatives representation is based on population size. The support of Congress is needed to declare war and appoint Supreme Court judges. Congressional committees in public can hold the executive to account e.g. Watergate.
Secondly there is the executive which in the US is the President and government. Unlike the UK, the executive is separate from the legislature. The President appoints his own executive, but is constrained by Congress for example in extreme circumstances can be impeached. In recent decades the powers of the executive have increased as Presidents increasingly bypass Congress, for example Nixon using Kissinger as a foreign policy advisor unaccountable to Congress.
Thirdly there is the judiciary, in the US The Supreme Court. Throughout US history the 9 judges have made historic ruling on issues such as slavery Scott Versus Sandford and abortion Roe Versus Wade. They are appointed for life so if vacancies come up during a Presidential term, this gives a President the ability to make appointments reflecting their own politics, which will influence if not determine Supreme Court rulings for decades.