The principle of 'separation of powers' proposes that the power to govern be divided between different bodies to avoid any one branch having all the power. Power is divided between a law-making body (Parliament), an executive body (Executive) and courts (Judiciary).
The Australian Constitution divides the power to govern Australia between the Australian Parliament (that makes laws for all Australians), the Prime Minister and ministers (who put the law into action), and the High Court of Australia and other federal courts (that make judgements on the law).
An example of the principle of the separation of powers at work at the national level is how laws are made and tested in the courts. The Executive (Australian Government) comes up with the idea for a bill (proposed law) which it introduces into the Australian Parliament. The Parliament (law-making body) then debates the bill and decides if the bill should become a law. If the law is the subject of a court case, the Judiciary (the High Court or another federal court) will make a judgement on the law or whether it has been followed.