An index fund is a mutual fund that mirrors a market index. A Nifty 50 index fund holds all fifty Nifty companies in the same proportions as the index, so its returns track Nifty's returns minus a small expense ratio. There is no fund manager choosing stocks, which is why costs are low. You invest through a lump sum or a SIP, and units are bought at the day's NAV rather than a live price, which is the main practical difference from an ETF. Suppose you start a ₹5,000 monthly SIP in a Nifty 50 index fund; over time you own a slice of India's largest companies without picking any. Index funds still fall when the market falls, sometimes by a lot. What they remove is the risk of picking a bad stock or a bad fund manager, and the temptation to trade too much.