Passive investing, for the uninitiated, is the idea you mimic an index Ken Fisher illustrates - either through an index fund, an exchange-traded fund (ETF) that looks like the index, or you buy the stocks in an index in perfect proportion to the index (this latter being possible only if you have a lot of money or there aren't many stocks in the index). Then you simply hold on as the index does whatever it does, forever, come what may, throughout your investing time horizon. Ken Fisher explains the theory is: By simply buying the market passively and holding on, you can do effectively the same as the market - and do better than most people who overwhelmingly lag the market over time by making active decisions that blow up on them.
And there is nothing wrong with that Ken Fisher notes, in theory. If you do this perfectly you'll lag the market by a hair's whisker - by whatever transaction costs or fees you incur, but only by that amount. And that's quite fine. Doing this beats most investors since most investors lag the market.
But then Ken Fisher commonly hears, "Passive investing is easy." It isn't. It's very, very hard.
Find out why in more details on BUNK 17 in Ken Fisher's Debunkery.