If in addition to the note, the borrower executes a mortgage pledging real estate as security for repayment of the note then if the borrower defaults, the creditor, who holds the note, will decide if it will elect to sue on the note or foreclose against the property. If the creditor chooses to foreclose and is successful in bringing the property to auction then assuming that the property is worth less than what is owed under the note, the creditor can seek a deficiency judgment against the borrower for the difference between the fair market value of the property and the amount owed under the note.
A deficiency judgment can result in the creditor filing restraining notices against the borrower's bank accounts, executing on other assets and garnishing a percentage of the borrower's employment income.
There are ways to minimize the chances of being personally liable for a judgment and and consulting with an experienced attorney can potentially save a judgment debtor from the wrath of a judgment creditor.