The Grinold-Kroner model breaks the stock market's expected return into three parts you can reason about: the cash it pays you, the growth of its earnings, and whether investors will pay more or less for those earnings in the future.
Stocks should pay more than Treasuries for the risk you take. When the gap is thin, you're being paid little for it.
Expected annual return for different ending P/E ratios and horizons, with everything else unchanged. Over short horizons, repricing dominates. Over long ones, income and growth take over.
Grinold and Kroner (2002). The model adds its parts together, an approximation that works well when each is small. Repricing is the annualized change from today's P/E to the ending P/E over your horizon. It describes the market as a whole, not a forecast for any single stock.