In recognition of the fact that vesting all Government power on one source could lead to abuse and mismanagement of power as well as a lack of accountability, the Founding Fathers saw it fit to separate the powers of the Federal Government into 3 branches.
These 3 branches had their own powers which were separate from the other and would allow them to be able to put a check on the powers of the other such that no branch would reign supreme and enforce its policies on the country unilaterally.
An example of this principle is the power of Congress (Legislative) to create laws. The President (Executive) can decide to veto the law by not assenting to it and the Supreme Court (Judiciary) can judge the law to be unconstitutional thereby voiding it even if the President assents to it.
