What It Means
Liquidity is how easily you can get in or out of a trade near the price you actually expected — without your own order moving the price against you. A liquid option has a tight bid-ask spread and steady volume; an illiquid one has a wide spread and barely any trades, so even a small order can move the price or simply not get filled.
How It Works
Three things drive it: how many contracts are actively traded that day (volume), how many are still open (open interest), and whether market makers are actively quoting both sides. Index options like Nifty and Bank Nifty near the current price are usually the most liquid contracts on the exchange — tight spreads, heavy volume, easy fills. Move away from the money, into a far strike or a lightly-traded stock option, and liquidity thins out fast: fewer quotes, wider gaps between what buyers offer and sellers ask.
Example
Say you want to sell a call option on a mid-cap stock that barely trades in F&O. The last traded price shows ₹8, but the actual live quotes are a ₹2 bid and an ₹8 ask — nobody's actually willing to buy at ₹8, that print is old. You'd have to sell at ₹2, or wait and hope a buyer shows up closer to your price. Compare that to a Nifty at-the-money option: bid ₹150, ask ₹151. You place a market order and get filled within a rupee of what you expected, every time.
Warning
Don't treat the last traded price as what you'll actually get in a thin contract — check the live bid-ask spread first. In an illiquid strike, the LTP can be stale by hours while the real tradable price sits far below it.