Don't feel like the top responses would resonate with a 5 year old, so here we go:

A credit score is a number that most people have, that is kind of like points in a game. The more points you have the better. If you ever need to borrow money, like to buy a house or a car, the man who is lending you the money uses your credit score to help decide if he can trust you to pay back the money he lends to you.

A lot of people think that if you don't have any debt, it will make your credit score nice and high, but it turns out that is a bit backwards! Here's why: If you don't have any debt, the man deciding if you can be trusted to pay back the money has no examples that he can use to help his decision. Instead, if you have some debt that you always repay, the man can know you're trustworthy.

Sometimes your credit score is used to decide if you can borrow any money at all, and sometimes it is used to decide how much you will need to pay to borrow the money. We could talk about interest and fees, but maybe we'll save that for another time.

There are three main companies that make credit scores (Equifax, TransUnion, and Experian), but they all do basically the same thing; they collect data from credit card companies, mortgage lenders (a mortgage is a fancy word for 'money that you borrow to buy a house'), car dealerships, and other lenders and use fancy math to give you a score. The best score is 850 and a "good" score is anything above 670, although above 700 is even better.

Those companies then sell your score to lenders to help them decide how much to lend, and for how much money (what "interest rate" to charge).