A mortgage default occurs when you are 90 days or more behind on your mortgage repayments. Late repayments will often incur a late fee on top of the amount owed which will continue to gather interest along with the remaining principal amount.
If you do default on a mortgage repayment you should try and catch up in next month’s payment. If this isn’t possible, and missing payments is going to become a regular issue, you need to contact your lender as soon as possible to organise an alternative payment schedule and discuss further options.
You may also want to talk to a financial counsellor.