What It Means
Delta is an option Greek that measures how much an option's premium is expected to change for every ₹1 move in the underlying asset's price. A call option's delta ranges from 0 to 1; a put option's delta ranges from 0 to -1.
How It Works
Delta also works as a rough probability estimate — a call with a delta of 0.30 is behaving as if the market prices roughly a 30% chance it finishes in the money. Deep in-the-money options have delta close to 1 (or -1 for puts), moving almost rupee-for-rupee with the underlying; far out-of-the-money options have delta close to 0, barely moving at all. At-the-money options sit near 0.5, which is why they carry the most time value and react most sensitively to changes in implied volatility.
Example
If Nifty is at ₹24,800 and an ATM call option has a delta of 0.5, a ₹100 rise in Nifty should move that option's premium up by roughly ₹50, all else equal. A far OTM call with a delta of 0.10 would move by only about ₹10 for the same ₹100 rise.