In the context of Bogle's statement (and the parent's comment), it was the stock market, specifically index funds.
It goes back to the realization that no investor can predict the peaks or troughs of the market, so getting out of the market is effectively trying to time it. One may be lucky and get out at the peak and then buy back in at the bottom, but on average you will lose money this way.
It goes back to the realization that no investor can predict the peaks or troughs of the market, so getting out of the market is effectively trying to time it. One may be lucky and get out at the peak and then buy back in at the bottom, but on average you will lose money this way.