How to read it
Start from the example
Each example is an NSE end of day file at least 30 days old, dated on the page. The box opens with the settlement price of the strike you pick, and the solver returns the volatility printed in the chain table.
Type a price of your own
The solver looks for the one volatility at which the Black 76 price equals your price. A price below intrinsic value, or at or above the most an option can be worth, has no answer, and the page says so instead of guessing.
Read the two charts
The curve shows the option price for each volatility, rising smoothly, so one price fits one volatility. The smile shows the solved volatility of the out of the money option at every strike of the chain, puts below the forward and calls above.
Why the smile is not flat
One volatility per expiry would give a flat line. Index puts usually cost more than the model with a single volatility says, and the smile shows it. The solving for implied volatility module walks through the method, and the historical against implied volatility module says what the number is a measure of.
What it leaves out
Implied volatility is the market's price for one strike and one date, expressed as a volatility, not a forecast of what the index will do. The settlement price of a strike that did not trade is set by NSE with its own model, so the volatility of a far strike mostly reflects that rule. The rate is a T-bill yield taken as the risk free rate, and the model prices European options on the forward; the page never shows live prices.