TL;DR An index fund doesn't pick stocks — it buys everything on a list (like the S&P 500) and charges almost nothing for it. Decade after decade, that "do nothing" approach has beaten the large majority of professional stock pickers.
The plain-English version
Regular ("active") funds employ smart people to pick winners, and charge for the effort. An index fund fires the pickers: it just replicates an index — S&P 500, total US market, total world — holding every stock in it, weighted by size. No opinions, no forecasts. If the market is up 8%, you're up ~8%, minus a fee that rounds to pocket lint (often 0.03%–0.10% a year — $3–$10 per $10,000).
You buy them as mutual funds or ETFs; the wrapper differs, the idea is identical.
Why "no effort" wins
It sounds wrong until you see the scoreboard: over long periods, the large majority of professionally managed funds fail to beat their index after fees — the longer the window, the worse it gets (industry scorecards routinely show ~80–90% of large-cap funds trailing over 15 years). Reasons: fees compound against you every single year, markets are brutally competitive (the pros are trading against each other), and this year's genius fund routinely becomes next year's laggard. The index doesn't have a bad year relative to itself — it is the benchmark.
This is why the standard boring advice — from people with nothing to sell you — is some version of "buy a broad index fund, keep buying, ignore everything."
The honest trade-off
Index funds guarantee you'll never beat the market — you are the market, fully exposed to its 30–50% crashes, holding its winners and its junk alike. You're trading the lottery ticket of outperformance for the near-certainty of not underperforming. Most people, most of the time, are mathematically better off taking that deal.
The common mistake
Buying an index fund… then trading it like a stock — selling the dip, re-buying the rip. The entire superpower is behavioral: it only works if you let it be boring.
Educational only — not investment advice. The index fund's edge isn't brains; it's the absence of expensive ones.