The Australian Constitution is the set of rules by which Australia is run. It describes the composition, role and powers of the federal Parliament. It sets out how the federal and state parliaments share the power to make laws. It also details the role of the executive government and the High Court and defines certain rights of Australian citizens, such as the right to religious freedom.
The Constitution was written for all Australians as it provides the framework for government. For example, section 51 of the Constitution sets out the law-making powers of the Australian Parliament, which include specific areas such as defence, taxation, currency and foreign affairs. If a member of the federal Parliament wanted to make a new law, they would need to follow the rules of the Constitution. If they did not follow the Constitution, they would not be allowed to proceed with making a law.
If there is disagreement about the rules of the Constitution, the High Court of Australia can get involved to make a final decision. The High Court interprets the Constitution and decides its meaning, as well as settling disputes between the federal and state governments. You can find out more about the High Court's role in these matters here.