The Financial Conduct Authority (FCA) is a group of grown-ups who make sure that banks, insurance companies, and other grown-up businesses are treating their customers fairly and not doing anything that could harm them. Just like how your parents make sure you always play nicely with your friends and don't hurt anyone's feelings.
The FCA checks that these grown-up businesses are following rules and regulations, similar to school rules that you have to follow to make sure everyone is safe and happy. For example, these rules could say that insurance companies have to explain all the terms and conditions of policies in a way that everyone can easily understand.
If the FCA finds out that a grown-up business is not doing things properly or breaking any rules, they will tell them off like a parent who scolds their child for misbehaving. They might also give the business a punishment, like a fine or even stopping them from doing any business at all. All of this helps to make sure that the businesses are doing the right things to keep everyone happy and safe.