The formula is expected return = risk-free rate + beta x (market return - risk-free rate). With a 4% risk-free rate, a 5% equity risk premium and a beta of 1.2, expected return is 4% + 1.2 x 5% = 10%.
The implication is strong: risk you can diversify away earns nothing, so an investor is paid only for exposure to the market factor. That idea underpins index investing and the language of alpha and beta that dominates performance reporting.
Empirically CAPM explains far less than it claims. Low-beta stocks have historically returned more than the model predicts and high-beta stocks less, which is one origin of the low-volatility-factor. Multi-factor successors such as fama-french-three-factor were built to patch the gaps.
Related: beta, alpha, modern-portfolio-theory