Okay, so you know how when you borrow toys from your friends, they might want to know if you're good at taking care of things? They might ask your other friends or your parents about you, to make sure you'll give the toys back and not break them.
When grown-ups borrow money, it's kind of the same thing. People who lend money, like banks or credit card companies, want to make sure they're giving money to grown-ups who are good at paying it back on time. So they will check on how well the grown-up has paid back money in the past, and how much money they owe right now.
That checking is called a "credit check." It's like taking a peek into a grown-up's toy box, to see if there are things they haven't returned yet or if they're really good at taking care of them. Based on what the credit check finds, the lender might decide to give the grown-up the money they need to buy a big toy (like a house or a car), or they might not.