The separation of powers created three distinct branches of government, each with a domain it is responsible for. The legislative branch, which is Congress (Article I), is meant to make laws. The executive branch, which is the president (Article II), is meant to be commander in chief of the armed forces, to be in charge of all the departments under the executive branch, for example, the Department of State and the Treasury Department, to make treaties, and to nominate various "officers" of the United States, as well as Supreme Court justices. The judicial branch, which is the Supreme Court (Article III), is to rule on matters involving the Constitution, on any federal laws, any maritime laws, or in any matter in dispute between two states. Separating these various powers of government was intended to prevent just one person, or a few people, from having too much power.
In the creation of this framework, the founding father also instated what we commonly refer to as "checks and balances." This was meant to ensure that no one branch of government could act completely on its own and gain too much power. Think about a partnership where there is a partnership checking account. The account can be set up so that both partners have to sign a check, preventing either from deciding to spend money without the partner's approval. The president can approve or veto any laws enacted by Congress. Congress can refuse to fund the military if it does not approve of the president's actions as commander in chief. The Supreme Court can strike down a law enacted by Congress. In each of these example, one branch acts as a check against the actions of another. And the balance of power amongst the branches is somewhat equally distributed.