Mr. Glennon presented a document on budgeting that warns about the dangers of credit card debt, especially compound interest. It explains that while credit cards may seem like a way to get things now and pay later, the interest charges can quickly accumulate over time. Compound interest works by charging interest on both the principal amount borrowed and on previously accumulated interest, causing balances to grow rapidly. The document urges careful consideration of interest rates and repayment plans before taking on credit card debt.