You have the opportunity to purchase an INR 99 voucher, which gives you an INR 850 discount on a one-way ticket, or an INR 199 voucher, which gives you an INR 1800 discount on a roundtrip. [Edit: Readers tell me you need INR 8000K worth of ticketing to be able to use it, something not stated in the T&C]
The vouchers are one-time use only, and generated instantaneously and delivered to your email. However, to use them you would have to ticket with the airline on the phone. All in all, a beneficial deal in case you decide to take it. And yes, you can use the voucher for bookings made till June 15, 2014 and travel till September 30, 2014. Near date bookings can also make use of this discount.