Self Browse :
If you fall below the tax bracket (i.e. your annual income is below the taxable limit set for the year), you obviously want to avoid a situation where you pay tax first and then try to recover it from the tax department later. One such case is Tax Deducted at Source (TDS) - fixed deposits and post office schemes in general deduct TDS on interest and pay it to the Government.
Form 15G allows you to declare to the Bank or Post Office that you fall below the tax bracket. You can use this for yourself or for your Hindu Undivided Family (if you have one). When they receive this declaration, they do not deduct TDS on interest thereafter. Of course, a wrong declaration by a person who falls in the tax bracket is a punishable offence!