Credit. It's a scary topic. Mainly because so many things can surprisingly ruin your credit scores, such as late or missed payments, applying for more credit cards, carrying high credit card balances, ignoring your credit report, and more.
As an adult, you have a credit report that summarizes how you've handled your credit accounts which are reviewed and scored. Think of it as your credit report is your new report card, and your credit score is your new GPA. A negative inquiry or delinquency on your report can drastically lower your credit score. This can take time to restore which you might think will ultimately put a hold on many parts of your life, like financing a car.