ICICI says you can expect the market to tank (go well below 4500). So, they are asking you to sell a call option @130. The amount you get is Lot Size x 130 - brokerage. For NIFTY, lot size = 50, so you get ~6400. For MINIFTY, the lot size is 20 and you can calculate accordingly.
However, if you take the call, you need to deposit a certain amount of margin money with ICICI. Call up your relationship manager to ascertain the exact amount and terms.
If, the market goes up, you lose, as the call option's value increases, much higher than 130. And you would have to end up paying the difference (new price - 130) x Lot Size. And then there's the margin call from ICICI.
You have a couple of options once you have sold the call -- when the market has tanked sufficiently, you can square off, by buying a call. Or, if you expect that the market will keep going down till the end of August, you dont do anything. The option will be closed automatically on expiry day (28 Aug in this case) and you get to keep the premium. And yes, what works for you is that option value decreases with time, so as expiry comes nearer, chances of the call going up is less.
Selling a call is similar to buying a put, though, selling is riskier, you have limited profits (~6400) but unlimited losses. Read about options, before you even venture into this.