Foreign investment review agency, also known as FIRB, is a group of grown-ups in Australia that checks and approves foreign investment in the country. Just like when you invite a friend to your house to play, your parents may want to check with your friend's parents before they come over. FIRB does the same thing, but with countries instead of friends.
When someone from outside of Australia wants to buy a big company or some important land, like a farm, FIRB looks at all the details of the request to make sure it won't hurt Australia. They check if the people buying are trustworthy and not doing any sneaky business. FIRB also wants to make sure that the things being sold to foreigners are important for the country's wellbeing and won't pose any problems for the Australian people.
If FIRB thinks that the investment is safe and won't hurt anyone in Australia, they will approve it. But if they have any doubts, they talk to the people who want to buy and may ask them to change their plan so that it's safer.
FIRB is like a superhero who protects Australia's interests and makes sure only the right kind of people can invest in the country. So next time you hear about someone buying a big piece of land or a company, remember that FIRB is checking to make sure it's the right thing to do.