A lender uses your score to detmine if it's risky to give you a loan.
A high credit score puts you in a "low risk" category—congratulations, you'll get a good interest rate on loans and pay less in the long run for your purchases.
A low credit score will put you in a "high risk" category—you'll get a high interest rate on loans and end up paying a lot more! Don't worry, you can improve your score with a little time and effort.