Another way of thinking about it is: on average, stock value over the long term (20+ years) is very likely to be in range of, say, 5%/year, plus or minus (actual number is not that important for our purpose here), after adjusting for inflation. If stocks have been on a recent runup, gaining, say, 50% or 100% over a period of a few years, then they are very likely to grow more slowly than average (i.e., revert to the mean) over the coming years. If you buy during period of inflated prices you will realize a lower than average return if you're holding over the long term.
Robert Shiller (Yale finance professor, of Case/Shiller housing index) likes to make the following analogy. Predicting the stock market is basically the opposite of predicting weather. With weather our short-term predictions can be fairly accurate, but our long term predictions are very poor. With the stock market it's the reverse, short-term predictions are worthless, but long term predictions are generally fairly accurate.
Robert Shiller (Yale finance professor, of Case/Shiller housing index) likes to make the following analogy. Predicting the stock market is basically the opposite of predicting weather. With weather our short-term predictions can be fairly accurate, but our long term predictions are very poor. With the stock market it's the reverse, short-term predictions are worthless, but long term predictions are generally fairly accurate.