When a check is bounced, it means that someone has written a check without having enough money in the bank. The payee will not receive the funds.
If you have $400 in the bank and write a check to the mortgage company for $1000, the bank will not send the mortgage company $1000; they will not even send $400. The mortgage company will receive nothing. The mortgage company’s bank statement will say NSF, which means “Not Sufficient Funds”.