Overdraft programs are really high-cost, short-term loans with quadruple digit APR rates. Until recently banks could automatically cover your transactions when there’s not enough money in your account, and charge an astronomical fee for this service without your permission.
The Federal Reserve Board has issued final rules which give consumers the right to OPT-IN to these expensive programs. This means that a bank cannot charge you a fee if you overdraft using your ATM or debit card, unless you explicitly sign up for their program.
Before signing up, remember that there are often much cheaper options for covering overdrafts.