In one line
The strike is the fixed price written into the contract, and the exchange lists a whole ladder of them around the current level so that a trader can pick how far from the money they want to be.
How it works
NSE lists NIFTY strikes fifty points apart near the money and further apart in the tails; BANKNIFTY uses a hundred. The ladder is not decoration — it is what makes an option chain a chain, and every number a trader reads on this desk is computed per strike and then summarised across the ladder.
Three words describe where a strike sits relative to the index, and they are the most-used jargon in the market. A call whose strike is BELOW the index is in the money: exercising it now would be worth something. A call whose strike is ABOVE the index is out of the money: exercising it now would be worth nothing. The strike nearest the index is at the money, and it is the one this desk quotes volatility and the straddle price from, because it is the strike with the most time value and the least intrinsic value in it. Puts run the other way: in the money above the index, out of the money below it.
The desk draws about ten strikes either side of the money by default. That is a count rather than a percentage on purpose — a trader watching one screen for six hours needs a stable number of rows that does not reflow when the index moves. What the desk STORES is a percentage band, because a fixed count would have meant a far wider band in 2008 than it does now.
What it is not
A strike is not a level the market respects. It is a number in a contract, chosen by the exchange from a fixed grid, and the desk's own tests of whether heavy strikes act as barriers came back negative. Where a strike matters is in what it costs and what it pays, not in what price does when it arrives there.