"French Australian" means someone who has parents or ancestors from both France and Australia. This person will likely have a mixed cultural background, meaning they have learned and experienced things from both French and Australian lifestyles.
Imagine your friend Lily, whose mom is from France and dad is from Australia. When she visits her grandma in France, she gets to eat delicious French pastries like croissants and macarons. When she visits her grandpa in Australia, she gets to go to the beach and play with kangaroos.
Being French Australian means you get to enjoy the best of both worlds - French culture and Australian culture. People who are French Australian may even speak both languages and have unique traditions and celebrations that mix both cultures together.