Many banks charge a fee if a minimum balance isn't maintained in a checking account. Frequently this is calculated in terms of an "average daily balance", a running total of where an account holder stands every day in maintaining that minimum average. But no bank provides a way for account holders to track it themselves, nor provide the metric they use. And, as The New York Times has noted, banks frequently don't calculate the average until the end of the month.
Ron Lieber co-authored that New York Times story, and here to explain more.