Willful failure to file means that a person knew, or reasonably should have known, that they were required to file an FBAR, and chose not to. The standard penalty for willful failure to file is $100,000 or 50% of the balance of the account at the time of the violation, whichever is higher, for each year that a required FBAR wasn’t filed.
In some cases, willful failure to file could even result in a prison sentence. (The same penalties apply for knowingly filing a false or fraudulent FBAR.)