The doctrine that a balanced constitution requires a proper separation between the functions of the executive (the government and civil service), the legislature (Parliament and other legislative bodies, such as local authorities or rule making committees), and the judiciary (courts and judges).
Basically, the legislature makes the laws, the executive implements them, and the courts enforce them.
But it is not quite so simply divided. Legislative power is often exercised in accordance with government (executive) policy, and the courts may (under the doctrine of precedent that operates in common law jurisdictions) make or at any rate develop existing laws. (Courts generally decline to make new laws from scratch, and rarely intervene where a substantive change, particularly in an area already covered by legislation, depends on changing social mores and so dictates the need for public consultation and parliamentary debate.)