The basic idea is that the three branches of government (judicial, executive and legislative) can keep each other honest. When one gets out of line, the others can bop it back down. For example, if the courts get too free in interpreting the laws, Congress can pass a new law. If Congress passes a law that the court considers unconstitutional, then the Supreme Court can slap it down. If the president breaks the law, Congress and the cour can stop him. In reality, it doesn't work exactly like that. The power of the executive branch and Congress has been growing.
The above post provides you with a great summary of the checks and balances provided for within our constitution. The three branches of government all have restrictions on what they can do without the involvement of one or both of the other branches.
Allow me to summarize, however briefly. Congress has the power to "check" the power of the President by overriding Presidential vetoes; the power of the Senate to approve all Presidential appointments, and the power of impeachment. Congress can further check the power of the Courts by the Senate's right to approve all judicial appointments, determine the number and nature of federal courts below the Supreme Court, and determine the number of seats on the Supreme Court as well as the power of Impeachment. The Executive may check the power of the Congress by vetoing acts of legislation with which the President disagrees. He may also appeal to the people, as did Woodrow Wilson for the Treaty of Versailles. The President checks the power of the Court system by his right to appoint all Federal Judges, including the members of the Supreme Court. The Courts may check both other branches by declaring their acts unconstitutional; plus all Federal Judges are appointed for life, and may thus not be removed because of philosophical disagreements with the other branches.
A full answer, including all the details of all the checks and balances is beyond the amount of space we have to answer questions here. Please follow the link for a very detailed discussion of all the checks and balances.
In general, though, the Constitution provides for a system of checks and balances by giving each branch of government some amount of influence in each other branch's basic function. For example, the basic function of the Congress is to legislate -- to create laws. However, the President has a check on this function because he (no women yet) can veto laws passed by Congress. Similarly, the judicial branch can strike down laws passed by Congress if the laws are unconstitutional.
By giving each branch some power to interfere in the others' basic functions, the Framers created a system of checks and balances.