The principle of Checks and Balances refers to the system whereby all three branches of the government (the legislative, the judicial and the executive) can influence, oversee, check, and limit the other. Through this system, each branch can respond to the action of the other.
The President, for example, who is the head of the Executive Branch, is responsible for appointing public Ministers, Consuls, Judges of the supreme Court, and other Officers, however, this authority is checked by the Legislative Branch which is responsible for confirming or rejecting those nominees.
Another executive power is to sign into laws bills proposed by Congress, or to reject them: when Congress passes a bill, it is sent to the President for signature. If the President approves it, the bill becomes a law, but if he or she vetoes it, the bill goes back to Congress where the members get a chance to override the Presidential Veto with a certain majority of votes, this is also an example of how the system of Checks and Balances work.
Furthermore, the legislative branch has the ability to remove the President from office in exceptional circumstances, and the Judicial Branch has the power to evaluate executive acts and to render them invalid if they conflict with the US Constitution.