Okay, imagine that you are playing in a team, like a soccer or basketball team. When you join the team, you have to promise to follow the rules and play fairly with your teammates and opponents. This is called an oath of allegiance.
In Australia, an oath of allegiance is a promise that people make when they become a citizen of Australia or when they take up an important public job like being a politician, judge or police officer. The oath is like a formal way of saying that you will be loyal to Australia and will do your best to help the country and follow its laws.
When you take the oath of allegiance, you have to say some special words. The words go like this: "I swear by Almighty God that I will be faithful and bear true allegiance to Her Majesty Queen Elizabeth the Second, Queen of Australia, her heirs and successors according to law. So help me God!"
This means that you promise to be faithful and loyal to the Queen and the country of Australia. It's like a very important promise that you have to take very seriously.
So, if you ever become an Australian citizen or take up an important job in Australia, you will have to make an oath of allegiance to show that you are loyal to the country and its laws. It's just like when you join a soccer team and promise to be a good teammate!