A notice of default is a letter sent to a homeowner telling them that they have not made their mortgage payments and that they need to pay for the missed payments, plus fees, in order to avoid foreclosure.
The letter will call making up for the missed payments “curing the default”.
A notice of default is required by law and will usually arrive after three months of missed mortgage payments. It shouldn't be a surprise since you know when you haven't been paying your mortgage and the bank will have been in contact before sending the default letter.