The Supreme Court has a major task of interpreting and defending the US Constitution. Because of this, the Supreme Court can check the actions of the President and Congress when necessary if their actions are not allowed by the Constitution.
The Supreme Court is the final judge in cases involving laws of Congress and the Constitution if a law is not compatible with the Constitution, the Supreme Court has the task of declaring it unconstitutional.
The President is the one responsible for nominating justices to the court and the Senate is responsible for voting its approval of nominations.
