The Judicial Branch

Some claim that it is within the proper role of the courts to settle disputes between the legislative and executive branches, but then who is it that settles disputes between the courts and another branch? Who is it that defines the proper role of the court?

Some claim it is within the proper role of the courts to decide the meaning of the constitution even when it applies to restrictions on government power, but what then restricts a part of that same government (the courts) from deciding the constitution to mean whatever it pleases, granting itself unlimited power?

Some claim that the courts interpret the laws for us because sometimes we cannot agree on the meaning of the laws, but if we cannot agree on the meaning of the law, how can we then agree on the meaning of the rulings of the court?