Republicanism means that a country doesn't have a king, queen or other ruler who is born into their position. Instead, the people of the country choose someone to be their leader. In Australia, some people think it would be a good idea to become a republic and choose our own leader instead of having a queen or king from another country (like England) as our leader.
This idea started gaining popularity in the 1990s, when a big debate was held about whether Australia should become a republic. Some people thought it would be great to have an Australian head of state instead of the Queen of England.
However, not everyone agrees with this idea. Some people think that having the Queen as our head of state doesn't really affect our day-to-day lives, and changing to a republic could be expensive and cause a lot of problems.
So, in summary, republicanism in Australia is the idea that we should have our own leader chosen by the Australian people, rather than having a queen or king from another country as our head of state. It's a topic that has been debated for many years, with some people thinking it's a great idea, and others thinking it's not really necessary.