A benchmark index is a market yardstick, like the Nifty 50 or the Sensex, that a mutual fund's returns are measured against to judge whether the fund manager is doing a good job. Every mutual fund is assigned a relevant benchmark based on the type of stocks or bonds it invests in — a large-cap fund is typically compared to the Nifty 50, while a mid-cap fund might be compared to the Nifty Midcap 150.
If a fund consistently delivers higher returns than its benchmark over several years, that's generally seen as a sign of good fund management. If it consistently lags behind, you might be paying active management fees for a fund that isn't outperforming a benchmark you could track cheaply through an index fund instead. Always compare a fund's returns to its own benchmark, not to unrelated indices or other fund categories.