Alpha is a risk-adjusted measure of active return on an investment.
The FF 3 factor model is emerging 2 classes of stock with CAPM to reflect a portfolio's theory.
r - Rf = beta3 x ( Km - Rf ) + bs x SMB + bv x HML + alpha
Alpha Coefficient can show that in an efficient market, the expected value of the alpha coefficient is zero. Therefore the alpha coefficient indicates how an investment has performed after accounting for the risk it involved:
Alpha_i < 0 : the investment has earned too little for its risk (or, was too risky for the return)
Alpha_i = 0 : the investment has earned a return adequate for the risk taken
Alpha_i > 0 : the investment has a return in excess of the reward for the assumed risk