A settlement the IRS reaches with those who can never pay their tax debts. An OIC determines the amount you owe and the schedule for payment. See IRS Form 656.
The IRS charges an application fee of $150 for processing and evaluating offers in compromise. In certain circumstances the fee may be waived or refunded. When submitting a lump-sum OIC you must make a down payment of 20% to the IRS. The IRS explains the rules here.