This is an explainer, not financial advice. For many ordinary savers, one of the most consequential investing ideas of the past half-century is also one of the simplest — and a recent Guardian piece on tracker funds is a reminder of why it endures.

What a tracker fund is

An index, or tracker, fund is a fund that simply mirrors a market index — a fixed basket of shares such as the FTSE All-Share in Britain or the S&P 500 in the United States. Instead of a manager actively deciding which stocks to buy and sell, the fund holds all, or nearly all, of the companies in the index, in the same proportions. When the index rises, so does the fund; when it falls, the fund falls with it. No star stock-picker is at the wheel.

The 'haystack' idea

The phrase comes from John Bogle, who launched the first index fund for ordinary investors at Vanguard in the 1970s. "Don't look for the needle in the haystack," he urged. "Just buy the haystack." His argument, backed by decades of data since, was blunt: after their fees are taken into account, the large majority of professional fund managers fail to beat the market over the long run. In any given year most active funds tend to lag their benchmark, and the share that does so only grows over a decade or more. If beating the market is that hard, the reasoning goes, an investor is often better off just owning it cheaply.

Why cost matters so much

A big part of the appeal is fees. Because tracker funds require little research and little trading, they typically charge a small fraction of what actively managed funds do. That gap may sound trivial, but compounded over decades it is anything but: a difference of a percentage point a year, sustained over a working life, can erode a meaningful slice of an investor's final pot. Trackers also offer instant diversification — buy one S&P 500 fund and you own a stake in hundreds of companies at once. They come in two main forms: traditional index funds and exchange-traded funds (ETFs), which trade on an exchange through the day.

The catches

None of this makes index investing a free lunch. By design, a tracker delivers the market's return — which means it also delivers the market's falls, in full, when shares slump. You will, by definition, never beat the index you follow. And there is a subtler risk: because most big indexes are weighted by company size, a handful of giant technology firms now make up an outsized share of them, so a "diversified" tracker can be more concentrated in a few names than it looks. Critics also worry about the sheer scale of the largest fund managers, whose index products now hold huge stakes — and voting power — across corporate life.

A balanced view

Proponents of active management argue it can still add value in corners of the market that are less efficient — smaller companies, or emerging economies — where diligent research may pay off. But for plain, long-term exposure to the stock market, the weight of evidence has made the humble tracker the default choice for many. Its enduring appeal rests not on a slogan but on arithmetic: own the whole haystack, keep costs low, and let time do the work.