Instead of a manager choosing stocks, an index fund simply buys all the shares in an index in the same proportion. If the Nifty 50 rises 10%, a Nifty 50 index fund aims to return close to 10%, minus a tiny fee.
Because there is no expensive research team, index funds charge a very low expense ratio, often under 0.2%. Over decades, that cost saving compounds into a meaningful advantage over most actively managed funds.
For example, a SIP into a Nifty 50 index fund gives you a stake in India's 50 largest listed companies in one go — simple, cheap and diversified. You accept the market's return rather than trying to beat it.