When a debit, check or other payment hits an account without enough money, the bank either returns it unpaid, which is an NSF, or pays it into overdraft. Your bank may charge its own fee, and a funder whose debit bounced may add a returned-payment fee under your contract.
Underwriters count NSFs on your statements. A few in a hard month can be explained, but a steady pattern tells a funder that the account runs too thin to carry another payment. Expect questions about each one.
If you know a debit will fail, call the funder before it runs. Repeated returned payments are often listed as a default, and a phone call costs less than a string of fees.