The executive is the part of government that has sole authority and responsibility for the daily administration of the country. The executive branch executes or enforces the law. The division of power into separate branches of government is central to the idea of the separation of powers.
The separation of powers system is designed to distribute authority among several branches — an attempt to preserve individual liberty in response to tyrannical leadership throughout history. The executive officer is not supposed to make laws (the role of the legislature) or interpret them (the role of the judiciary). The role of the executive is to enforce the law as written by the legislature and interpreted by the judicial system.