If you had a reduction in your income or had financial difficulties but now you are getting updated this option lets you pay the amount that you are behind in a period.
It is to arrive at a payment plan to catch up with the arrears. There are several ways to negotiate a Forbearance agreement. Generally, what is requested from the bank is that you can make your regular monthly payments, and that the balance of the overdue payments is paid off in additional monthly payments for a period, usually twelve months.
Most of the temporary modifications or "trial modifications" consist of "Forbearance" agreements in which the person is asked to make partial payments, without this implying that it is a modification to his mortgage.