Okay kiddo, so there are these four companies, which we call the "big four". They all have names that sound like people's names, but they aren't really people - they're big, powerful businesses that do something called auditing.
Now, auditing is kind of like checking someone's homework. When a big company like the one that makes your favorite toys or the one where your mom or dad works has lots of money and important information, it needs to make sure everything is correct and legal. The big four companies help with that, by checking the company's financial statements and other documents to make sure all the numbers are accurate, that the rules are being followed, and that there aren't any risks that could cause problems in the future.
These four companies are really good at what they do, so they get hired by a lot of companies all around the world. They make a lot of money doing it, too! But sometimes people worry that because the big four companies are so powerful and important, they might not always be fair or tell the truth. So, they get checked by other people too, to make sure that they're doing their job right.
So there you have it - the big four are four companies that help other big companies make sure they're doing things right. And just like your teacher might check your homework to make sure you're learning the right things, other people check the big four companies to make sure they're doing their work properly.